Well, i woke up three days ago and had a white circle on the inside of my lower lip. how can i tell of it's just a canker sore?

Apthous Ulcer: Cancer sores, or apthous ulcers, are usually the result of trauma or stress-induced activation of the herpes simplex 1 virus, to which most have been exposed. There are many types of herpes simplex viruses, so this does not necessarily indicate an STD. Most are painful and can last from 7-10 days, similar to a cold virus. Painless ulcerations should prompt evaluation by a physician.

Canker sore: A canker sore will,heal in about 10 days. The best way to determine if this is a canker sore or not is to see a dentist. If this does not heal in 10 -14 days then it would be very important to see a dentist and determine what is going on.