re: the pain and discomfort: Esther is giving good advice here. Aches and pains are common following prolonged computer work and it is unlikely to be true epcondyiitis. However the problem can escalate out of hand and relatively simple measure like Esther is suggesting can often nip the problem in the bud. You could also register and and have a look at this website that is run by the National injury insurance scheme in NZ. The author is a physio and and ergonomist:

ACC DPI Programme > Home

RE: the shaking. This may not be anything to worry about. the contraction you are doing is called an eccentric contraction so when you lower your hand against gravity the muscles that normally lift your hand in the upper side of the forearm are working. It is common for some people to have a shake in these muscles both at the forearm and the calf muscles at the ankle when performing eccentric contractions. It can be worse if the muscles are tight or overused and fatigued but it usually is just a funny artifact of a normally working nervous system.You can often learn to suppress the shake by concentrating on improving the smoothness of the movement as an exercise but there really is no need to do this. As long as it isn't interfering with your and and finger control I wouldn't be concerned.