It is still possible that the area you have discovered on your inside lip is related to some trauma to the area, either from the biting habit that you describe or from accidentally biting your lip while eating. These sores can take a while to heal because the tend to rub against the teeth and they also tend to become irritated again with eating.
Another possibility is that this is a canker sore. Canker sores are painful ulcerated areas inside the mouth which have no clear cause, although they may sometimes be associated with stress. Canker sores are not contagious and they cannot be spread to another person. They are also not related to cold sores, which are caused by the herpes virus and are contagious. There is no particular treatment for most canker sores, as they tend to get better on their own over a few days to a week. Sometimes, pain medications are necessary and, in the most severe cases, other prescription medications may be indicated to help them heal more quickly.
This is definitely an issue that you should talk to your primary care doctor
about. They can examine the area in your mouth, confirm the diagnosis, and help you decide whether treatment is needed.