Right arm: weakness, muscle pain, elbow pain, constantly. What could it be?

Tendonitis. Tennis elbow or lateral epicondylitis, often presents as pain and/or weakness in the arm. Most of the symptoms are felt on the outside part of the elbow, down through the forearm. Lifting pain be painful, as can use i'd the hand or arm. Ice, otc nsaid's and braces are good initial treatments. If pain persists for more than 4-6 weeks, go see a board certified orthopaedic surgeon.