Please help: my husband said his penis is burning after we had sex.

We had sex tonight, no condom or lube, and he did ejaculate inside of me. He said immediately after his penis started to burn. We are both STD free and have only been with each other. (Yes, I'm 100% positive). Does anyone know what could cause this? 20 minutes after he said the burning was gone. I have not experienced any symptoms such as burning or yeast like infections. We are currently going through fertility and just finished 3 rounds of Clomid, if that makes any difference. Thank you in advance for your help.