My boyfriend and I have been together 3 years but 2 of those years have had to be long-distance due to university, so I know the struggle and I wish you well. From my personal experiences, the first 6 months of separation are the hardest, after which, being sensitive to each other's schedules and plans almost becomes ritual. However, it doesn't appear she thought too seriously about remaining in a solid relationship... everyone knows what can/tends to happen at freshers and her mentality was to completely chill out herself without considering the situation at the event (i.e., many guys looking for a flirt, and that she is certainly NOT single). It's obviously OK to enjoy yourself and have fun, but being in a relationship does bring with it a certain type of responsibility which she completely neglected. Personally, I wouldn't trust her. I'm sure she knew, from your conversations, that you'd be anxious about the long distance and that the first few months in particular would be very important for you both in determining what to do next.
No matter what, if you truly care for someone, you don't suddenly forget about them and their feelings in a spur of a moment!
You must remember it will never be the same again as things like this cannot simply be forgotten... 'the cracks can be filled with glue but they will forever be visible' kinda thing.