The first thing I think you should do actually si go down to the doctor and get yourself tested for STDs! The LAST thing you want to do is find that you've picked up something form this guy friend of yours only to pass it on to your boyfriend because I think that would just about finish this whole thing of wouldn't you say? SO get yourself checked out for STDs before you do anything else and remember that unprotected sex isn't JUST about pregnancy. WHile you are there I do suggest you ask if it's too early to try for a pregnancy test/checkup. You might be able to find out yet if you are pregnant and if you are then what your options are. The sooner you find out the better because the wider your options will be and the more time you will have to discuss them with the right people. If you put it off because you are scared then you are shortening the time you have to approach the people with whom you need to discuss it as well as limiting your options. SO get yourself down to the doctor for both those reasons and put your mind at rest one way or the other.
If you find anything is wrong then make sure you get all the information you can and check out all the possibilities.
With regards to the cheating I thin you really should come clean. These things have a nasty habit of lying low and then biting you in the rear just when you think they won't - not to mention that you will have to live with the guilt of having done this and that won't pass unless you try to make it up to your boyfriend. I know you might not want to do this, but just imagine how it would feel if it came out and he found out through someone else. He would feel angry, hurt and humiliated and it would so much worse because it wasn't even you who told him. MOreover he would have a clear sing that you lied to him.
As it is you were drunk. You weren't in control of your actions. Now that in itself is not good, but you only make things much much worse if you chose to compound your position by lying to your boyfriend. If you lie then you are making the conscious decision to betray him and that may actually be worse then what you did when you were drunk because it is premeditated and he will see that. It will look like you are only looking out for yourself and that you can't be trusted.
At least if you come clean then you are showing him that you can be trusted to tell the truth when you fuck up and in my book I know that's worth a lot. I trust people who admit to their mistakes because it takes guts and I know that I can at least rely on their honesty.
YOu are going to have to be very careful about how you do this though. you want to make sure he is relaxed and receptive and you want to make sure that you reassure him a lot and let him know that you love him very much and were very stupid to do what you did. You're going to have to eat a huge piece of humble pie and put up with an angry reaction - probably an over-reaction - because he will feel like he has been betrayed. You will have to try to reassure him that it meant nothing, that you barely remember it and that you would never do it in your right mod. You're also going to have to give him some reassurances that it won't happen again in the future because he's going to want to know that this was a one off mistake. Try to listen to his feelings and not overstate your own. He needs to feel important ad he needs to feel that he is first when you tell him. If you go on about your feelings then it will sound like you are making excuses or being selfish.
Things will not be the same for some time afterwards. You will have to make it up to him and you must be prepared to show some humility of your boyfriend really means something to you. You'll have to swallow your pride for a while. But in the long run it's worth it, isn't it, if you get to keep him, rater than lose him? I know it will be hard for you and at some point you must make sure he isn't continuing to punish you forever for one mistake, but bear with it and I think you can work this through. If he truly loves you then once you give him some time to get past the anger and the feelings of betrayal then he will see that you fucked up and he will forgive you. You just have to give it time.
Good luck, and if you want more advice or just someone to talk to then sign up for an account so you can PM me.