what is it?

Please refer to photo of my lower lip - http://imageshack.us/photo/my-images/580/lipap.jpg/

I am 29yr old male and non smoker. I had this blister for around a month now. It started red, then grew and become white. It also has a pointed edge. At one point I started to apply ad-muc ointment to it and it went away but then reappeared after a few days. I also tried lemon juice, but it wont go away.

Any suggestions as to what might be and how I could heal it? it doesnt ache or burn so far.