Randomly sensitive to the formula??

Anyone’s baby become randomly sensitive or allergic to their formula after being on it for a long time??

Baby girl just got/is getting over a stomach Bug which I assume was in fact a stomach bug because it wasn’t only strange loose BM’s but also projectile vomiting. Doc said to put her on soy formula for about 5 days and if she seems better by then to give her the regular formula again. Tuesday was her first day back on her regular formula and until today she has been fine.

Today she had 4 BM’s and the last two were nasty🤢😬 i breast fed her until 5.5mo but she was always supplemented with at least half formula due to my supply never meeting her needs. She will be 10mo on the 21st so about 5ish mo now being strictly formula fed🤷🏻‍♀️

This is sooo weird to me. Both of my kids attend daycare at the center I work at and we are currently on an every other week work schedule working 5.5hr shifts. We were home from March 20 till end of August and the kids have both been sick since going back which I figured would happen but after being home all week on my off week I’ll be headed back Monday and omg I can’t imagine her being sick againnnnn and having to take off. I’m so lost on what this is.