First time we had sex and I’m feeling low

I started dating someone new for the past 2 months. We are two hot heads but I guess there’s a lot of passion.

Today we had sex for the first time and I can’t help but feel sad. Even after the sex, and still now even a few hours later. It’s not a type of sadness I felt before say with an ex where it was because I realised how much I cared about them but a sort of lost sadness.

Does anyone else feel the same when they sleep with someone new for the first time and is this normal? Could it be something else thag I haven’t realised?