It is common to get fever blisters on the lips during Ramadhan. There are two simple solutions for this annoying little problem.
First, make sure you are drinking enough water. One of the signs of dehydration is dry lips. When the skin is dry, tiny cracks appear on its surface. This allows harmful bacteria and virus to enter the body. Keep lips moisturized by getting plenty of water 8-8 ounce glasses per day. Applying lip balm on the lips helps because it helps keep moisture from leaving the skin. Just be sure not to ingest it while you are fasting.
If you already have a fever blister, here is a cure that is natural and works well. Apply a dab or two of fresh, plain yogurt on the affected site. Use a yogurt that does not have flavorings or sugar. The bacteria in the yogurt heals the fever blister within a day or two of applying it. Apply after breaking the fast if you lick your lips a lot. and try to break the habit of licking your lips. You can even go to sleep with yogurt on because it dries and will stay on the lips. Although yogurt-lips do not look very pretty, neither does a fever blister.
The yogurt-fever blister cure is one that I always use for this problem and it works great. Of course, I try to avoid the whole thing by getting plenty of water, applying a protective barrier (i.e. olive oil, lip balm, etc.), and washing my hands often.