>>1
Unless you've got a disease (fairly common, actually), it just means that your gums aren't in great shape. Brushing more often and especially FLOSSING will keep them healthy and will strengthen them in a relatively short amount of time (I'd guess a week or two, but I'm no dentist).
I went through a period when I was a kid where I was almost afraid of brushing my teeth because they bled, which was because I didn't brush regularly. And, embarrassingly enough, until only about two years ago, I wouldn't floss because they bled and it hurt every time I did it, which again was because they weren't very healthy because I hadn't been taking care of them.
tl;dr: Brush more often and FUCKING FLOSS (at least twice a day).
Oh, and you might want to use a soft-bristled brush until your gums are in better shape.