What is a bad ache to the right fore arm?

Depends. Certainly tennis elbow is a common cause but there are also types of tendinitis closer to the wrist that can be associated with forearm pain. Nerve problems as well. Any of these problems will be part of an rsi type situation i.e. Repetitive stress or strain a good history and exam will tell. Best of luck.
Forearm pain causes. Typically forearm pain is actually arising from the outside of the elbow. The pain then traverses down the forearm. This condition is called lateral epicondylitis or tennis elbow. It is a common condition and is treated non operatively with a wrist splint, activity modification, exercises icing and patience.