My DS was doing something similiar when we were still attaching (I use only EBM now). It turned out to be a milk protein allergy and I had to exclude all dairy from my diet. Initially it was thought to be reflux. I'm not saying that this is the case with your LO but just something to keep on the radar. Funnily enough I noticed it after binging on chocolate at Easter time. If it is a dairy issue then it takes 2-3 days to get out of your milk but it can take 2-3 weeks for little tummies to heal.

I don't think changing to forumula will help as FF babies get reflux too and formula can get comlex if it's a milk allergy as you need a prescription formula.