Help me. !!!

Can someone please tell me what this is?? It doesn’t hurt hurt but when i touch it it’s a little sore... i just noticed it last night after sex and my bf “finishing” on my butt cheek. But it may have already been there prior to sex... and I’m just being overly paranoid. Because i don’t remember scratching or falling. 🤔🤔 crazy question... can sperm burn your skin?