Hi, welcome to gayspeak.
From what you tell us, I wouldn't be TOO concerned.
You say that you're a virgin, so I would suspect that if anyone had tried penatrative sex not only would you have been alerted to it at the time but you will probably have been aware of some pain in that area, perhaps a little tendernes and bleeding for a few days afterwards.
As well as the issue of someone having sex with you without your knowlege or consent, perhaps you're also anxious about your status as a virgin. Many guys are and prefer to loose their virginity to someone they love and trust - or at least have the hots for. I was on Matt Damon's waiting list and Brad Pitt's before that but I realised I would probably be in my late 90's before my turn came - with either one!
If you're still worried about the physical aspect of things, maybe you might want to get checked for STI's. I don't know what the situation is in the USA but here in the UK we can go to a Sexual Health clinic at any hospital in the country.
Maybe next time you're at a party alternate every alcoholic drink you have with a soft drink. To be blunt, moderate the amount of alcohol you drink. Check up on what is considered to be safe drinking levels for you. Here in the UK for a man it's 2-3 units a day 14-21 units a week. A unit of alcohol is equivelent to 100ml of ethenol alcohol and every unit will take one hour to metabalised by the liver. So, one pint of ordinery strength beer or larger = 2 units and 2 hours to get rid of it from your body.
If it's practical and emotional support you find yourself in need of, you've come to the right place!
I've been using this site off and on for about three years and find it to be a really friendly and supportive set of forums.
I'm sure you have nothing to worry about!
Hope that helps, and good luck!