Could be gum disease like gingivitis .. Or could just be an infection caused from an unsanitary toothbrush or could be an adverse reaction from your toothpaste. Try changing brands maybe. Often bad reactions are common to toothpastes with added whitening, mouthwash etc. Be sure to change your toothbrush at least every 3 months. In any event if this is troublesome perhaps it's time for a checkup At your dentist.
