When you say you lost a substantial amount of weight, was this healthily and did you need to, and are you now underweight as a result or a healthy weight? This makes a difference, or can make a difference, to one's fertility. If you needed to lose weight and you lost it steadily and healthily then I don't see it affecting your fertility in a negative way unless you're now underweight.
I would download Fertility Friend (by far the most accurate and helpful app I have found out of trying many), buy a BBT thermometer (you can buy one from Amazon for a fiver, make sure it reads four digits instead of three as a regular thermometer would read) and start temping from either the first day of a new cycle or when your period finishes. I temp vaginally because it's more accurate for me. You can temp orally but if one night you sleep with your mouth open or talk in your sleep for example, it can make your temps off, temping vaginally won't do this, although try to get the thermometer in the same place each time whether orally or vaginally.
Temp the same time each day before you do anything else on waking, or at least try to keep it at the same time each day. This isn't always possible, sometimes we sleep in or wake earlier, but if you feel your temp is way off then there are online calculators which can adjust it for you if you feel it needs adjusting.
I would use the cheap strip OPKs instead as these work fine without as much of the cost, you can buy them on Amazon too cheaply for quite a few. I usually use three of these each day just before the time I would usually ovulate as sometimes you can ovulate earlier than usual. Use fmu, mid afternoon and evening and hopefully this way you won't miss your lh surge. Fmu doesn't usually give positives for many, but I have had some positives with it and it's best to test throughout the day so you have much less chance of missing your surge. The test line has to be as dark or darker than the control line, and once you see a positive you could ovulate that day, or as late as three days after. So I read, the older you become, the closer to a positive you will usually ovulate. I typically ovulate on the day or the day after a positive OPK, but everyone's different.
Once you ovulate you will likely see a clear shift upwards in your temperature, but sometimes it can take a few days for this to happen. Fertility Friend usually pinpoints ovulation once you have three days of increased temps.
Check for cervical mucus and cervical position - usually when you're at your most fertile you will have a high, soft and open cervix (this also depends on the time of day you check, so try to keep it at the same time each day as your cervix can change position and feel throughout the day), and you should see egg white cervical mucus or watery mucus around ovulation. If you don't get this, then it's worth taking some evening primrose oil if you can (providing you don't have an allergy to it, it doesn't interact with any medications you take and you don't have any other medical reason to avoid it), drinking more water also helps your body to produce fertile cervical mucus. Drink at least 2L of fluid each day, not including caffeinated drinks.
Try to limit caffeine intake too, and alcohol consumption (bear in mind drinking alcohol can alter your temperatures).
Coming off birth control can screw up your cycles for a while afterwards, but if you're not having regular cycles by about six months then see your doctor, or sooner if you prefer. Even if your cycles don't regulate, this doesn't mean you won't get pregnant. But it will likely mean you may need an earlier dating scan if it's hard to pinpoint ovulation. The app can also change the dates depending on the information you put in and which setting you use. Start with the advanced setting as it's easier and usually correct, it isn't always perfect and can get it wrong but most of the time it's fine.
Listen to your body too. Sometimes you will see ovulation pains, sometimes they may be quite general and centred, sometimes they may be to one side, sometimes they will be both. Sometimes you will notice you want sex more when you're close to ovulation too.
The app itself looks like a nightmare if you're unfamiliar with it, but it doesn't take long to get used to and it is actually simple to use and read. All you have to do is enter in your temperature each AM, check and enter in what your cervix is in feel and position and what your mucus is up to, and whether you have a positive or negative OPK. If you haven't any idea when you ovulate I would suggest starting OPKs the day after your period finishes at first, to until you start seeing a pattern of when they start to darken and when you ovulate after temps confirm. Once you have this more pinpointed, you can start the OPKs closer to when you suspect you should ovulate. My earliest day has been CD10, the latest day CD14, so I tend to start CD7 just in case my body decides to give me positives super early. They're so cheap that it doesn't matter if they're wasted. The Clearblue tests are so expensive you can't do that with those unless you have plenty of cash at your disposal, if you have then by all means use those! You could always use both - use the cheap OPKs until they start to become darker and then use both or switch to the CB instead.
Also, don't be surprised if you see a dark OPK immediately after your period or immediately before it, as we can have a mini surge in lh around those times. So probably best to ignore those, as they should fade out and then start to become darker later on in your cycle. Thought I should add that in, as it has many women confused when this happens
