Baby spits up breast milk, not formula. Why?

girlmomx3
I pump and bottle feed & formula feed Neosure to make up for what I can't supply. Whenever I give my milk she spits up, sometimes quite a bit. When fed formula she's fine. The first ingredient in her formula is nonfat milk so I don't think it's a dairy thing. Is there something else I could be eating that's causing this or any ideas what else her issue could be?? TIA!