Woke up this morning with a blister on the edge of my lover lip near the corner. It's a single slightly raised blister with a kind of dimple in the middle and looks to have clear fluid inside. It doesn't exactly hurt, even when I touch it, but I can tell it's there. There was no warning, no tingle, it just appeared sometime while I was sleeping last night. Over the last week it's been getting cold and my lips are slightly chapped. What could it be?