Formula help!!!

I'm really confused on what to do about my babies formula she spits up a lot and seeems to get acidy with the similac advance, we are currently trying spit up similac per pediatrician request and it's been great and she has been holding it down better now yesterday she spit up and it was a huge nasty clump, and this morning just now feeding her she projectile vomited like 6 times at once soaking my whole shirt her onesie and the bed and I'm 100% sure her stomach is empty now, do you think she might be lactose intolerant?? How do you tell :( she is 4 months old and her weight was in a lower percentile this month from her last check up and I think it’s because she has a hard time keeping formula down