What Causes Painful Sore In The Inner Part Of Labia?

I have had an open sore on the inner part of my labia for three wks. It started as a small irritated area then it seems the friction from my panties caused it become a full fledge open sore. It hasn't grown and is quite painful. It has not shown any signs of healing. There is no pus-like discharge. It is smaller than the size of a dime and is flat in appearance