I think I cheated on my girlfriend

Why are you scared to ask your friends?
One of two things is true: either it happened or it didn't. The longer you put this off the harder it'll be.

If you did harm, there is a window to address and make amends that will close very soon. Unfortunately friend, this may be one of those things you have to force yourself to face head on with some urgency and let what happens happen.

If there is ANY hope of rebuilding trust, anything at all working against that trust rebuilidng will be devastating.
I asked my friends, and they're saying that I didn't. I don't know what to think man.
I just have the faint memory of it, but I'd have guessed that one of them would have some notion of it.