supplementing formula

I was breastfeeding and pumping. she hadn't gained weight by day 5, by day 9 she lost ounces, so we started formula. I had given her bottle with bm once before and she took it. but I got a different bottle because it seemed that the first was too much for her little mouth. It has now been 2 days since adding formula, I still pump to give her BM and try to get her to bf, but it seems she's eating way more often , every hour and 1/2, during the day, she goes about 4 hours during night. my question is.. is it normal for her to keep wanting to eat that often? she spits up a lot too so this tells me she was way too full. just need some input.