Your girlfriend has been getting suspicious that you're cheating on her because you are. You need to do something to make her feel more secure. Why not agree to wear an electronic monitoring collar around your neck.
"But I don't want this," she'll say. "I just want to believe that I can trust you."
"And I'm demonstrating that you can," tell her. "You don't ever have to turn this on. But I'll know that you could any time you wanted to check up on me. And you'll know that I know, so without ever even having to turn it on, you'll be guaranteed that I won't cheat. Because I'll know that I could get caught. Blammo, our trust is guaranteed!"
"But I won't ever turn it on," she'll say.
"Yeah but how do I know that. See, I have to be faithful just in case."
"Okay," your girlfriend will say. "I guess it will work."
You and your girlfriend will kiss and part ways. Around eighteen minutes later, she'll turn on the monitor and discover that you're at your ex-girlfriend's house. You're really fast, and really stupid if you thought that she'd buy that "you don't ever need to turn it on" horse hockey. Anyway, pull your sweatpants back up because she's on her way over.
Happy Wear an Electronic Monitoring Collar Around Your Neck To Prove To Your Girlfriend That You're Not Running Around On Her Day!