My upper left lip just suddenly swoll up. what could it be?

Allergy or infection: Sudden swelling without pain is likely an allergic response. If pain is present then infection is possible. In the absence of trauma and pain, an uncommon allergic reaction called idiopathic angioneurotic edema is the most probable cause. It is worrisome but not life threatening. If it does not resolve with benedryl then seek medical attention.

Lip swelling: Any number of things can cause lip swelling. The most common is injury from blunt trauma, biting, or burns. Less common are reactions to medication or foods or inflammation of the small salivary glands that line the inside surfaces of the upper and lower lips. Social activity, i.e. Kissing. Is a common cause. Abscessed teeth are probably as common as injury. Dental injections may also do it.