Constipation?? Or what?

I started giving my baby 3oz of formula a week ago since 2 was not enough for her. Ever since then she has been having weird bm. It use to be almost every time I changed her diaper there was poop. Now this last week there has been 2 days, not in a row, that she has not went poop or only went once. She burps more than usual and seems to spit up a little more often, and she has been farting a lot. She is on emfamil newborn. I'm wondering if I should switch her to gentlease? She also started taking medicine called nystatin for thrush the same day I upped her to 3oz so I'm wondering if that could have anything to do with it since it says a side affect could be nausea and upset stomach.