I was sitting down with a couple of friends and I jokingly said to one of them “heck you”. She then said “you’re retarded”. I was not offended by this and was fine. I then said “ur moms retarded” (I’m sure we all know the ur mom joke by now and I don’t need to explain it). She then got super pissed at me and said her mom was dying. I had no idea anything was happening with her mom at all. She never told me. I then did something and came back and sat down with another one of my friends (that saw what happened). She told the other girl to chill out but then she got mad at her because she has bipolar apparently. She also never mentioned this. AITA?