Sounds like a cold sore or fever blister.
* Small, painful, fluid-filled blisters on a raised, red area of your skin, typically around the mouth
* Pain or tingling, called the prodrome, often precedes the blisters by one to two days
Does that sound like it? You can check with your pharmacist for something to help the pain. I've had these I know they're no fun. I've used Abreva for it.
How long have you had it? If it's too painful, gets bigger or you just want to check in with your doctor that's not a bad idea either since we can't see it.