Awww. I'm sorry you're feeling bad. Constipation sucks.
I had it a LOT at the beginning of this pregnancy. Drinking extra liquid can help, but that's often not enough for pregnant ladies.
Miralax is totally safe during pregnancy. (Approved my my brother who's an MD and my OB.) I take a half dose each evening and it keeps things moving. You might want to take a full dose the first two nights and then switch to a lower dose. Keep in mind, it will likely take a few days for things to move on, but that it will work.
If you keep taking a little each night, the constipation shouldn't come back (at least, not until you're much further along and then my brother says you just up the the dose to a full dose at night).
It's not a stimulant laxative so it can't cause any permanent changes in your body, it just keeps more water in your intestines. That means you could drink it every day for your whole pregnancy and your body would still function normally when you stop taking it.
Make sure you're drinking 8-10 cups of non-caffinates, low calorie liquid a day when you're using it or you'll get dehydrated.