My girlfriend is still friends with her ex?

Long story short, my girlfriend's ex is part of her like friendship group sorta thing, and when i go out with her and her other mates who i get along with he's obviously bound to be there, anyway last night we all went out and she seemed to talk to him alot more than i was comfortable with, this confused me because when me and her first started seeing each other she said he treated her like crap in their relationship and she didn't like him as a friend or anything, now she's saying they're mates etc. Anyway we had a little tiff about it in town and he was asking her ohhh you alright etc, we made up, i was meant to be staying at hers but said i was gonna go home because of this and she flipped, i apologised and went back to hers at the end of the night etc. Anyway in the morning i was on her phone playing angry birds (lol) when i just so happened to stumble across her message inbox to see texts from her ex from a few days ago, nothing incriminating but just seemed to bother me because there were texts from her going remember to text me happy birthday and everything when you get back from travelling xx, meh and just yeah seemed to bother me. I cant tell her who to be friends with but have i got reason to be a little annoyed?