You might've just bashed your nose slightly in the night. Problem with the nose is that there are a lot of very delicate veins behind a very thin layer of skin, so it bleeds very easily.
As long as you're not waking up with blood dribbling down your face I'd say there's nothing you really need to be worrying about.
When I was five I bashed my nose on my bunk bed railings and my nose bleed wouldn't stop. Turns out I'd hit a really week spot on my nose (I hadn't hit my nose that hard, wasn't painful) and I was going to have to have it cauterized (sp?). I still get them occassioanlly, and some really nasty ones where all of a sudden I've got blood just dripping out of my nose.
Think yourself lucky :P