Bf penis burns after sex

The last couple times me and bf have sex( unprotected) and he cums, his penis burns. Any ideas why? Is it me? We usually go a couple of rounds. We were both virgins, we lost our virginity to each other, i was his first kiss and I know he wouldn’t cheat and I haven’t

Update:

We also have anal sex but always do that after vag sex and he always washes his penis if we go from butt to vag. Also he pees in between rounds