It could be a cold sore or a blister or ulcer. Was it associated with any fever? Has it occurred earlier in the same spot? If so, it was possibly a cold sore. If not , any injury/ stress/ acidity could've aggravated an ulcer. Either way, continue applying the ointment, take a b-complex medication, and avoid touching or picking at it for the next few days. It would be advisable to show the blister to a dentist for clinical exam. Normally ulcers and blisters take upto a week to heal. The blister weeps, a scab forms and eventually the scab falls off to fresh, healed skin. Unfortunately there's not much you can do until this process is complete. Avoid sharing glasses, cutlery, kissing, any other contact until it is healed to avoid spread of the lesion.