Bigboy, screen names can be used by more than one person, and some are pretty common nicknames.  That in itself means nothing as far as proving your girlfriend is doing anything at all.  So, this leaves us with two possibilities.  1)  You are imagining/mistaken that she is surfing these sites.  2) She really is surfing these sites.

   You don't say what kinds of sites these are (dating sites, porn sites, webcam or social sites), and since you were surfing these sites too, does this mean you were cheating on her?  I'm sure your answer is no.   Surfing is not cheating, though surfing dating sites would definitely seem to be a prologue to infidelity.

    You need to take a hard look at yourself and make sure you aren't overly suspicious.  You also need to figure out why you are discussing this with strangers anonymously, instead of directly talking to her about this.

    Given the fact that you were in love with her in childhood (or through high school), didn't see each other for thirty years, then became emotionally involved after reuniting, I would guess that you have idealized/romanticized her.  How much of what you think and feel about this woman is real, and how much have you projected onto her and yourself?

    Good luck on figuring all this out.  I hope you two talk it out and you replace any false ideas with the truth.