what y'all think

before I make my son an appointment. he's 11 months and has been on similac sensitive but this past week he been throwing it up on and off especially at night and his poop been more smelly,yellow,stringy..he does eat more solids now and drinks water too..and it could be a number of things like a stomach bug,teeth,or what if he's ready to be switch off the bottle to regular milk... has anyone baby done this and once y'all switch to regular milk the baby felt better?