Sorry if this is not a delicate question...but why does my cat smell like sour milk?

And no, he hasn’t been in the sour milk (and I don’t have any), but I am really concerned he ate something that is affecting his digestion. I only ever give my cats moist cat food, dry cat food, and cat treats, so what could this be? I gave him a tiny bit of milk from my cereal bowl yesterday…could that be affecting his, ah, emanations from the rear?

Should I be worried?