Formula allergy?

My 8.5 month old son is fed and formula fed. We recently decreased the feeding and increased the formula and have noticed quite the increase in spit ups an hour or two after his feedings (4-10 spit ups after a formula feed). I realize it is normal for them to spit up, especially with how active he is on his tummy after eating (there's no keeping him still, I've tried), but I would have thought it would occur shortly after eating, not hours later. The spit up is foul smelling and it just seems to be constant through the evening (he gets formula for his 2 mid-day meals). Does this amount of spit up seem normal? He doesn't seem to mind it but I'm concerned he's losing so much of his food. He is on Similac Advanced.