Thank you for your question and welcome to Just Answer.
If the tooth that you are looking at is the second molar, then one possibility is that you have an unerupted wisdom tooth that is covered by the gum behind this last visible tooth. The pain is from bacteria and food particles that have percolated under this gum, far away from your ability of clean the area. A gum infection is starting and causing the pain.
This could also be a classic gum infection on the last tooth you see. If you have any periodontal bone loss around that tooth, then the same bacteria and food particles have migrated into the deep spaces under the gum and possibly under the fork in the roots of that tooth causing the pain.
If there is no decay or fillings on this tooth then the most likely reason for the pain is as explained above.
You probably should start taking some antibiotics that your dentist can call in for you to a local pharmacy and while you are talking to him, arrange an appointment to have the problem properly diagnosed and treated.
For now you can manage the pain as follows:
A very good way to control dental pain using over the counter medications is to switch between taking 400 milligrams of ibuprofen (e.g., 2 tablets of Advil), and 1000 milligrams of acetaminophen (e.g., 2 tablets of extra-strength Tylenol) every 3 hours. Be careful not to use these medications if you have had previous adverse reactions to either, or if you have had gastritis or peptic ulcer disease in the past, or if you have liver disease.
In addition, you can try "Toothache Drops" available at your pharmacy. This is clove oil (eugenol) which is very soothing to an exposed tooth. Place a drop or two on a cotton pellet and place on the tooth.
I hope you find this information helpful. If you have any concerns regarding this issue that I have not addressed, please let me know and I will get back to you with more information.
George McKee DDS