maybe you lack vitamin c or you have neglected your mouths health
Probably 'Gingivitis' (gum disease) or malnutrition. See a dentist soon.
Make sure you're not developing Pyorrhea, it's nasty. I had a friend who had it and it was gross.
it could be from a lot of things. if you changed eating habits, have a new toothbrush, dont floss enough/too much, etc. Consult a dentist :)
Could be that or infection, you might want to invest in a Water Pik and use it, a whole new level of cleanliness.