Stress ball causing issues

My ex-boyfriend and I went to a college summer bash yesterday and had a lot of fun playing games and coloring and just really enjoying ourselves. One of the activities was to make a stress ball. Hours after the event had ended, we went on a walk and about an hour into the walk he tried to take my stress ball and I said no, then he broke my finger and left with my phone and keys. We’ve been broken up for two weeks and when we hang out, he claims to have forgotten we broke up and start touching and biting me. We were best friends for months before we dated for four months.