Why do I feel this way

me and my SO (me 18yo ex 20yo) broke up about 2 months ago, we were together for a year and 7 months.

we still live together because he has nowhere else to go. tensions have been very high between us but I genuinely thought they were getting better. I had a heartfelt conversation with him and he agreed to be civil. honestly things had been going great but this is where my issue comes in. he hadn't walked around shirtless since the break up until today. guess what I see on his back, fresh scratch marks clearly from rough sex as well as hickeys. he was pretty much showing them off put his shirt on the second he made sure I saw them, made eye contact with me n everything. later he asked his friend (who is staying with us for awhile) if he was fine staying home alone so he could do something tonight (heavily implying it was sex) right in front of me. why is he doing this and why does it hurt so bad. I've been actively avoiding relationships and anyone who flirts with me since the breakup but he can just move on this quickly?