Rats fighting...?

Question
I own two female rats (Moxie, a plain hooded rat and Ponyo, a hairless, who is slightly smaller) that are a bit less than a year old. I've had them since August. They were even in the same cage at the pet store. They get along just fine, and both are well-behaved when I handle them. However, today I found what looks like a somewhat large bite near Moxie's neck, probably a day or two old. It doesn't seem to be hurting her at all, even when touched- accidentally, of course. Neither rat seems nervous or aggressive. They don't seem to be fighting, but I don't know how else she could have gotten that bite. I've checked both rats, and there isn't anything else on either of them. I'm not sure if I should get another cage and separate them or not, or if I'm overreacting. They're acting perfectly normally, but I'm scared that something is going on that I'm not noticing and that one of them will get seriously hurt.

Answer
Nah, don't separate them. This is totally, 100% normal. They argued over something - or perhaps not at all, and were just playing - and one of them got a little scraped up. Unless they really explode into a frenzy of arguments, leave them be, they're fine.