Lactose intolerance?

My baby is 4 months old and is on similac pro advance formula and has been pretty much since birth although I was combi feeding for about 7 weeks... She's always had tummy issues but the past couple days I feel it's been getting worse.. we use gripe water and the cocyntal medicine for gas and tummy upset but it doesn't seem to be working as good. I did bring this up to our dr but he basically didn't even entertain that she could be lactose intolerant... If your baby is, how did you know and what did you do to for sure rule it out? And if you think it could be something else please give me advice! I hate seeing my girl in so much pain all the time 😔