if you actually can't breathe through them, and they are every 15 minutes those are labor contractions!
braxton hicks contractions wouldn't be every 15 mins, they would be more sporatic and not so strong that you can't breathe!
I'd go in and get checked if you haven't already!
The rule of thumb is, if you have 4 or more with the same consitancy in an hour, it's labor contractions you need to call and get checked!
Sounds like bh contractions - but keep timing them and noting how far apart they are. If they get stronger and get closer - I would call your doctor. Better be safe than sorry.