This is difficult to diagnosis without seeing it. Some thoughts, some maybe out in left field. One, the tooth could be fractured--that sometimes happens even if you do not have any fillings present, it could be a gum issue, and then again it could be root sensitivity. I am sorry for the generalities but if it continues you need to see your dentist.
sounds like enamel erosion/toothbrush abrasion. the enamel protects the underlying dentin, which is VERY sensitive. by the gumline, the enamel is very thin, and does not extend under the gumline moe than a millimeter, if that. when you brush too hard, for a time, one of two things (or both) can happen--the tissue recedes, and/or the enamel gets worn away. either way, yo're left with a VERY sensitive area on the tooth. there are products/treatments for this, but you need to see a dentist. I'd try something called "super seal" first. no drilling- it just seals the dentin and may need to be repeated a few times. ease up on the rigorous brushing. most people should not use an electric toothbrush for this very reason. ask your hygienist how to brush correctly with a regular SOFT bristled toothbrush, and don't let her try to sell you anything. good luck