I get this. Oddly enough, I read somewhere that it might be carpal tunnel related. I'm not sure if it is deferring the pain to the forearm. This odd sort of carpal tunnel pain can also go to the upper arm, which it does for me. So, I started using a wrist brace when I sleep when I am getting that pain. After doing it a few days, the pain is gone (I did notice improvement the first day). I figure when I sleep I tend to bend my wrists too much, putting strain on the carpal tunnel.