Is it ok for a commited girlfriend to be out way past 2 am with a single guy?

Ok, I'm in a serious relationship with my girlfriend for two plus years now. It's been the best relationship of my entire life, she's told me of her's as well. Lately though, I've become very concerned. We've recently moved to a new city and in an effort to make friends she's been surfing the strictly platonic section on craigslist. Well, lo and behold some guy contacts her and they've hit it off immediately. Now, twice this week she's been out until past two am. She tells me that nothing's happening, or could happen. And I've no reason to fear, but when I'm out till three am with a girl hanging out laughing and drinking I call it a date. Am I right to be so god damn worried/angry, or am I really overreacting?