Is ex mucking me about? or tryiing to keep me around incase something goes wrong?

my ex girlfriend called it off after 2 happy n funny years.... the reason was she hasnt been felling herself for a while... possibly another lad involved....she has already ensure me there is not...but she invited me out the other night to a fair had a wicked time both of us...lots of laughter etc...bought her a baloon as you do :P but now no texts except after fair saying i had a good night thanks.... but recently got an ivite for her birthday i dont no wat to do?? is she playing me? keeping around or.... need some advice from either lads or ladys thanks