I'm really upset/confused by this... she went to prom with her ex?

this girl and i like each other. we've known each other for a while and 2 weekends ago had our first kiss. i'm only the third guy she's ever kissed so i think it means something. people i trust and know they wouldn't joke told me she really likes me. well i found out last weekend she went to prom with her ex (they go to a diff. school than me) and i'm just really pissed. she dumped him about a month ago and she even told me she didn't wanna even kiss him by the time they broke up. he put up pics and tagged her and they were all over each other, not kissin or anything but still. and besides tagging her name he re-tagged her as "cutest girl in the world". she hasn't commented on any of the pics but ppl are all posting stuff like awww ur so cute together and the guy seems to be lovin all the attention he keeps commenting back like "i know;]" and "thanks!"