It does sound like you might be pregnant, but don't freak out, take a test and see what the answer is. It is better knowing for sure than not knowing at all.
If you find out that you are pregnant, take a deep breath and go see someone that can help you, like at a crisis pregnancy center, and see what they say.
So take a test and come back on and tell us what's going on and whether or not you are pregnant.
Don't keep saying that you are stupid and then doing nothing about it. If you know that it is wrong, than stop. A guy who is willing to cheat on his girl friend to get some free sex is not the sweet caring guy you think he is. I understand that you are in love with him, and I know that's a really hard place to be, but try to think about it aside from the way you feel about him. Even if you ended up getting married to him someday, if he was willing to cheat on his girlfriend he'll be ten times more likely to cheat on you.
I really hope that everything works out for you, and don't forget to update us, ok?