What Causes Sore Blisters At The Base Of Penis?

I have about 7 sores on the base of my penis. They are scabby and look as if they were pimples that I scrached but are not. The diametre is about the same as a kernel of corn. They rise about 1cm from the skin. They are not full of puss but do sting when touched. My girlfriend does not show any signs of this on her genitals. Help!?!