Hi,
This is actually a torn frenulum. Its no big issue, make sure you keep it clean under there (topical cream like Canastin helps). Mine has torn a few times, doctor said just to leave it (tho it was a little painful, and in the end, i actually cut the last bit with nail clippers and its been fine since), apparently on some men it tears the whole way and many circumcised blokes don't have a frenulum at all anyway. Sex it now better again.
It could be a dermatological issue but a torn foreskin is not a symptom of any STD.
So what could have caused this? There was no rough sex.