These are always fun. Which arm (plucking or fretting)? What part of the forearm? Does it extend tot he fingers?
In the absence of this information I'll go out on a limb and make a guess that it's the the plucking arm. I believe the least understood and most prevalent issue for CGs stems from tension in the playing arm and pressure from the edge of the guitar against the forearm just south of the elbow. This will affect the ulnar nerve, which starts at the spine, makes up the medial brachial plexus, runs down the arm, through the elbow, and into your pinky, ring, and side of middle fingers. Tension and pressure can cause pain in the "funny bone" and into the forearm. Rest, perfecting a relaxed technique and, as BB says, bourbon will alleviate. Pick bourbon OR the NSAIDs as they're both crappy for your liver and kidneys. Google "nerve glides". Avoid bench pressing and push ups.
Whatever catastrophe or dynamic equilibrium this will eventually lead to will be a mathematical not a moral phenomenon. - A Fryer