Elbow, tendon, muscl. It is not clear at this point. There are different things we consider. Sometimes this can be tennis elbow. Is the lateral side of your elbow tender. We see this in tennis elbow. It can be worse when you resist dorsiflexion of the hand. The biceps muscle attaches about the elbow and the biceps tendon is actually in the shoulder, thus this tendon is not involved.
Elbow pain. Hyper extention of elbow can cause pain to anyone. You are probably injuring your elbow with some activity you do everyday. Try to analyse what you are doing and which arm it is. Even working at the computer with the mouse can cause pain. When you find the cause then try to rest the affected area. Ice packs help. Change of position at work or at computer might help. Look for the cause.