Sperm allergy?

i need some insight. i’ve only ever been with one partner and we’re still together. we’ve always had safe sex and back in april we decided that it was okay for him to ejaculate inside of me. every time it’s happened, everything has been fine, except for the last four times. as soon as he does it and pulls out, my vagina instantly starts to burn on the inside. it burns and it feels like it’s starting to swell. it always happens at night so i sleep it off and it’s gone in the morning but it burns so bad. it only burns when he does it in me, not when he pulls out and it hasn’t happened until recently. any idea what could be happening?