Why would he say something like that and act like this?

Friday seniors had to go to the Auditorium to get yearbooks and sit through this whole big presentation anyways i had to sit beside my ex (Forrest) He started talking to the guy beside him and i started to talk to my friends in front of him and he kept reaching across me messing and talking to the guy beside me and my cousin who was in front of me was playing around and said "well you're a guy" and Forrest budded in and said "woahhh...i know for a fact that she is not a guy" and that just really surprised me...why would he bud in and say something like that,and he kept talking about other girls to avery really loud so i could hear and when i pulled out my phone to text someone he was watching me...what does this mean? if it even means anything at all.