What if I told you I had sex with my girlfriend's 50 year-old friend behind her back?
Why would you jeopardize your relationship to have a fling? Did you tell her?
No. I didn't feel like that was necessary
I think she'll find out eventually.  It's always better to come clean rather than let them find out without you telling them.  You might still be able to save your relationship if you do so.
I don't need to tell my girlfriend. I treat her good in other ways so she shouldn't care.
It's good that you treat her well, but cheating is on another level. If you forget her birthday, you can make it up to her. But cheating is far more destructive -- think about how you'd feel if the roles were reversed.