So a couple of days ago I noticed I had a small blister type sore just below my knee about the size of a pen head and thought nothing of it then one to two days later my leg was hurting in the same spot so I went to look and the blister had grown into this sort of flat/blister sore and was now the size of a 5c coin (Australian money). I didn't touch it or anything and it grew by its self, it is red around the border of it and stings really bad if I touch it and the skin pulls away. What is going on ? Had anyone else experienced this? Does anyone know what it is ? *sorry about the blurry photo but I couldn't get a good one*