This could be an enlarged salivary gland
or cyst, but it sounds most likely like a mucocele
, or a mucous retention cyst.
These can form when a tiny salivary gland duct in the lip is damaged or injured, obstructing the flow of mucous and causing a cyst. Unfortunately, it won't spontaneously resolve and may need to be surgically removed. You should see an oral surgeon. Good luck!