AITAH for telling girls bf I slept with her?

Alright I (35M) started talking to a (35F) recently about a month ago. She never told me she was dating anyone but when I asked said 'single'. This weekend we were hanging out and things led to bedroom. On my way home, turns out she blocked me on all social media and phone #. I had a friend check her social media and it said 'in relationship with Derrick.' I searched his name and found him and sent him all the evidence. She unblocked me and called me an AH for telling and showing him evidence. Reblocked me, my friend said I should be just let it go. AITAH or should I have let it go?