You might want to go to your gyno and get checked. Could be something a bit more serious like endometriosis. Good luck and feel better soon!
its normal for air to get into the vagina during sex sometimes. Just push it out. The sex gets back to being great once you do. When ever I'm having sex with a girl, and I notice her vagina starts ballooning with air, I tell her in a sexual way "push my 'penis' out", or "push the air" out. When they do, not only does all the air come out after a few strokes, it feels incredible while they are pushing.
Yeah he sounds like a penis
Dump him. Telling you it's in your head is gaslighting (no pun intended) and a form of abuse. Also him not stopping after you've asked is a violation of your consent and a form of sexual abuse. No wonder you don't want sex! He sounds like an abusive as****e. Get out of that relationship ASAP.