What could cause a rash all over your lips?

Depends: Dry cold air is a usual cause of chapped lips that may be construed as a rash. Sun exposure may affect the lips more than other skin and may also activate cold sores. Deficiency of B vitamins is an other cause. It is not feasible to provide a more definitive opinion without examining the "rash". For good health - Have a diet rich in fresh vegetables, fruits, whole grains, low fat milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.