Why is my ex acting like this?

So Im a freshman and I dated this extremely hot senior Mathis for about 8 months. He dumped me because he said that I was too young? like really what the hell, after 8 months you realize oh wait she's too young. He said that technically the things we were doing were illegal and he told me he really loved me but it just doesnt feel right and he was crying when we broke up, so i was like hella confused and hella heartbroken about it but I was like its ok maybe he will wait a couple years and we can get together again? idk so two weeks later he asked out this girl Alexia who is 1 ***** YEAR OLDER THAN ME! like wtf, and hes like showing her off in my face. What do you think his deal is cuz i am so confused.