I'm sorry Emma - it sounds awful - especially not knowing what, if anything, happened.
If you can't get over the idea that maybe something happened, why not get yourself tested for all the usual STDs. A clean result might help you move on?
Like some others here, it doesn't sound to me like drinking a little bit is working out for you - even if this is the first unpleasant incident in 9 months, it's still one too many.
Have you thought about stopping completely?
D