There are several possible causes for a white painful sore (lesion) to appear on your tongue. The best thing to do is to return to the dentist for the appointment tomorrow and have him/her evaluate the lesion. It would be thoughtful on your part to call the office and tell them about the sore and ask to shorten the appointment time since you are reluctant to have any further treatment due to the sore.
The lesion could be an aphthous ulcer. It could be trauma from biting your tongue while you were numb. I could have been caused by inadvertent contact between your tongue and a dental instrument, the dental handpiece or the curing light used to harden a tooth-colored filling.
For the moment, the best treatment is warm salt water (1/2 to 3/4 teaspoonful of table salt mixed with 6-8 oz of warm water) rinsing and/or hydrogen peroxide rinsing. Avoid any spicy foods or mouthwashes containing alcohol. You can take Tylenol, or Ibuprofen (Advil, Motrin) to help relieve the pain. There are also some over the counter rinses sold in pharmacies for mouth sores.

Thomas B. Krull