Breastmilk VS Formula

My baby girl is a month old. Initially I was exclusively pumping, so while I wasn't directly breastfeeding she was getting breastmilk. I got sick a little over a week ago and while I was sick I couldn't get any milk out while pumping, now that I've recovered I am slowly getting my milk back in, but we've had to supplement with formula.

I have no issues with formula and giving it to my baby. The issue is that she seems to spit up more when I give her breastmilk now. I've gotten next to no spit up after formula feeds but quite a bit from breastmilk feeds. Am I doing something wrong? Should I give up on pumping and move to exclusively formula?... I just want to feed my baby and do whats best for her....