Hi, mandimoo!
I've had this a lot in one corner of my mouth recently. I'm not sure what causes it, but to help it heal, try mixing a bit of Sudocrem with Vaseline and applying it before bed (seriously attractive!) and again during the night, then constantly applying Vaseline during the day. It will heal within a day or two providing someone doesn't make you laugh and open your mouth too widely. :o)
When it has healed, use an old toothbrush to (gently) exfoliate the area every couple of days, moisturise, and keep up with the Vaseline so it doesn't dry out. It's all worked on mine.
Hope this helps. x