By any chance were you eating something scratchy like potato chips? If not, it could be a gum disease, which means a trip to the dentist is in order. Until you get there, try swishing with salt water to ease the pain.
gums could be sore due to a varied number of reasons. the most important one is plaque/calculus deposits on teeth which cause gum infections and thus lead to bleeding of gums. the other reasons may be any food impaction between teeth or newly errupting teeth(ex:wisdom tooth)