How do women always know?

I have been with my girlfriend for almost 4 years, and we have a son together. Now, I am no saint, and there have been times I have hooked up with other women, I can't help it at time, some women are just so sexy, and most of them don't even care that I have a son and a girlfriend. But whenever I do fool around, my girlfriend always knows... It's so weird... she always knows, no matter what I do. THe slightest change in my behavior, and she always says, "who are you f**king now?". I always deny it, because I would rather have her think she is paranoid. But that doesn't seem to work, because whether I cheated or not, she will just go into Angry Cat mode. Anyway... HOW DO THEY ALWAYS KNOW WITH NO PROOF!!!