You should get an allergy test
done, either a skin test
or RAST, to know what you are allergic to. Contact dermatitis could be due to tooth pastes, mouth cleansers, spoons, cups, glasses, food items, cosmetics, shampoos, etc, basically anything that comes in contact with your lips.
Another possibility is eczema. Herpes infection is another possibility if there is a history of kissing or sexual contact (can get aggravated in presence of vitamin deficiencies, dry cracked lips, infections etc).