AITAH for sleeping with my ex husband while he has a girlfriend

I 24f was previously married to my ex husband 33m for 5 years. Things ended due to our different interests we kept in touch staying freinds for our 2 year old son.

My ex is In a relationship with his girlfriend for a year. I met her and she was of the bat rude and straight up bitchy she didn't like me. I had spoken to my ex since her behavior was upsetting my son he agreed and they had arguments about it. He grew distant from her and he started spending time with our son and we grew close and we ended sleeping together. It just happened at first I regretted it my ex and I have spoken and he's planning on telling his ex.

So reddit am I the AITAH