What Cewsbaby said. Dye concentration varies SO much its crazy! And it doesn't matter if it's the same brand, and expensive brand, same box etc.
Also, there isn't a world of difference in those in my opinion. You probably won't see a huge difference over a day anyway, especially when you think it can take three days for hCG to double!
Cramping doesn't have to mean anything untoward. I had so much cramping with our daughter I didn't suspect I was pregnant and only tested because we were planning a night out. With my first child the line was incredibly faint and I was a couple of weeks overdue af.
The only thing you can do is try to avoid worrying (which I know you won't be able to do, in reality), test tomorrow or the following day as this will give you a clearer idea and if you feel there isn't the progression you would like to see, book in to have a beta done.
IF it isn't happening, then there isn't anything you can do except give yourself time to grieve and recover. Sometimes things don't pan out, they haven't for us five cycles in a row now and it is awful, you experience a whole load of emotions which are hard to cope with but you find a way because you have to. You don't get over it, you learn to deal with it over time.
But hopefully this won't be the case for you and things will be ok