Drink a three litre diet coke. Don't let the little burps through. Save it all for the Big One.
You let out a burp so grand it leaves you gasping for air. And though it is impressive, it's still clearly within the realm of the mundane. Although, now your teeth are very gritty, and when you scratch at them, you find little bits of dark reddish stuff. It's gritty and crumbly if dry but when wet it sort of smears. It's definitely not blood. You're not even sure it's organic.
In general, I'd focus on trying to breathe specially or maybe vomit, assuming it's a breath weapon or similar.
If that didn't yield anything I could go off of, I'd maybe try messing with my gut, suspecting maybe it has to do with resilience, specifically something about being full of gravel or similar.
Finally, I'd probably start sniffing things to see how I felt about eating them, then trying to eat unusual but not dangerous substances to see if it's some kind of ingestion power.
Breathing specially...well, you try gasping and huffing and puffing and generally acting like an idiot, but nothing happens.
Vomiting, on the otherhand, yields interesting results. You find you're able to do it at will, and rather then a mess of stomach acid and the sandwich from lunch, you upchuck a solid lump of reddish material about the size of your head. It looks like thick mud or clay that squirms around a bit before hardening into a solid reddish ceramic.
Hold a lit match in front of my mouth next time. Also, what Irony suggested about being Kirby. Try to capture and analyze a breath sample during a burp.
Your breath doesn't seem to be flammable, nor does it seem strange when you capture it in a pickle jar and take a better look at it.
In... oh.
Hm. Eat something, most likely. Brush my teeth and see what happens? Spit, too.
Brushing your teeth yields tiny particles of reddish substance in your sink. Eating something seems to have no effect.
I would bite organic and inorganic material for a long period each to ascertain if my teeth now possess alkaline or acidic properties
Your teeth seem to be 100% normal. Also, biting spatulas hurts.
Exhale on a flexible but solid object like a straw, test for any signs of petrification.
You are Not a Basilisk.
Feel free to build off each others discoveries. I'll let you know when you've figured it all out. Good work so far, though.