aha! got to love the internet. it contains a plant enzyme called bromelain. it's a chemical that's incredibly similar to one of your own stomach juices.
synthetically produced Bromelain is used most commonly as a meat tenderiser as it breaks down proteins, so basically what you're feeling isn't the effect of acid. it's the effect of a natural enzyme eating your flesh. like a fruit zombie.
leaving the pineapple overnight with air circulating around it helps apparently, as does drinking water while eating it.
I'm not gonna stop eating it, but I will go a little more easy on it knowing it's basically digesting my cheeks lol.