Boyfriend problems (I think)

Okay so I love my boyfriend, he is a good guy and we get along. The only thing is that one of his best friends is his ex GF. He went out with her when he was 17 for a 8 months or something and now he is 26 .. Theyve been friends since which i wouldn't mind .. Howvever they have a weird relationship, he isn't gay acting and neither am i .. Ive also had a Gf for 3 years before .. She is always asking him to buy her drinks, and tells EVERYONE new we meet that he is her ex BF .. He kisses her on the forehead when he see's her which i don't like and one time he had his arm behind her when they were sat on stools talking.. I just don't like it .. To the point it hurts where I feel I'd be better alone, although I wouldn't .. Advice please