<human>: Can you give me a shortened version of the text?

I've been in a LDR with my girlfriend for a bit over a year. We're very happy together and I'd never even entertain the idea of cheating on her, or at least that's what I used to think. Last night was my 21st birthday and unfortunately due to work commitments she couldn't come (she came up earlier instead when she could get time off of work).

Now I'm not the biggest drinker, mainly because I don't enjoy the cost/aftermath. Normally I'll have a few drinks to get buzzed and keep it at that. Being my 21st though I had people buying me shots all night and ended up black-out drunk by the end of it, passing out after having my friend carry me back to my bed. 

This morning I woke up and everything seemed fine (besides my headache). Later on though I was told that when I was drunk very late in the evening I kissed an old friend of mine who showed up by surprise to the party. I have absolutely no recollection of this at all and can't verify if I've done it or not myself so I'm going entirely off of what I've heard. The news made me feel physically ill all over again and I've been trying to remember it all day with no success.

Besides being disgusted with myself I've been trying to figure out where to go from here and how to tell my girlfriend. Since she lives over 1000 KM away it's a bit hard to sit down and do a face-to-face. What's the best way to go about this, and how do I even explain it when I'm not sure what happened myself?
<bot>: Been told that I kissed another girl at my party, can't remember any part of it and not sure what to do/how to tell my girlfriend.