Allergy?

My daughter had this but not as severe when she was a newborn. Pediatrician looked at it and said she was fine and to continue using non scented gentle products on her skin. She has had this off and on for like 3 months and now I’m thinking she has got to be allergic to something right? She has always spit up a lot and on formula for spit up per pediatricians suggestion. Sometimes she can get a small rash around her bottom but only a handful of times since she’s been born so I can’t tell if it’s a milk allergy or what. She doesn’t have any problems eating either. Thoughts??

(Her 6 month WC is this week so I will ask then)