What Causes Sore Patch On The Arm?

I have a sore patch on my left arm. It starts from about an inch above my elbow and finishes about 3 inches before my wrist, and the soreness is on the underside (i.e the part that doesn't usually get tanned). It's extremely sore and sensitive to touch, or if anything brushes against it, but there is no swelling, no rash, no redness, aboslutely nothing. It just looks like any normal arm! I have had this before in other parts of my body, and my mum also gets it sometimes, but we just can't seem to find anything about it!!!