Hi creaturewithin,what i mean is sometimes when your gf is starting to climax she may tense up,in a way worried she may loose the feeling if she relaxes and it can give you a feeling of your chest caving in /hard to breathe.
This will be because she is holding her breath without even realising it.
I hope this reassures both you & your gf if in doubt tho she really should see her gp.