Late On Wednesday Night After Coming Home From My Gun

Late on Wednesday night after coming home from my gun club, I had an itch on my left hip (where it meets the buttocks). I scratched it gently (like anyone would scratch any itch, and then suddenly my skin and muscles in that area started to spasm in an excruciatingly painful manner.
This morning it happened again (the spot is still itchy and tingling) and I scratched far more gently than anyone scratches an itch, but it still happened. Since then it has happened twice more. Once in the middle of the day when I forgot about protecting the area, and once again when I laid down in bed and rubbed against the mattress when turning over.
Each time it seems the spasms and pain get worse and affect a larger surrounding area (i.e. down my left leg and up my back). Even the first event was excruciatingly painful.
What condition could be causing this?