An STD is an infection and you can get one in your mouth if you go chomping on something that carries the infection, so you need a trip to the doctor pronto. Fortunately, they can be treated pretty well with antibiotics. Might not be a bad idea if you dragged your sex partner along for a test too. Gingivitis carries those sort of symptoms too, so although it's not an STD, whatever you've got needs some treatment.