Actually, I wonder if it isn't a weird hormonal reaction. Men don't have much oxytocin in their systems, but right after orgasm, they experience a BIG rush of it. This is a hormone commonly thought to increase bonding in people (the "cuddle" hormone). Since you and this girl aren't serious and you know it, you might be having some weird reaction to the oxytocin rush? I dunno, the timing is what's making me think this.

How the 'Love Hormone' Works Its Magic ? WebMD