Why would the muscles in my body jump involuntary. It does it all the time but is much worse when I'm tired. Sometimes it is even painful. It's bad enough at home where there isn't anyone around to see me do it but out in town is embarrassing.

Could you elaborate a little more? Is it an eye twitch or your arm suddenly jumping? When you boil it down it is usually, but not always, your nervous system misfiring. It could think that it needs a reflex repsonse or a move request got rerouted or sent without permission. The problem is that it is REALLY hard to say what causes that. It is really something that a neurologist needs to look at.

Please Google 'clonus' and read some definitions and watch some videos, and if you have any more questions, ask back.
In the legs it often is combined with complaints of 'restless leg syndrome,' for which people used to take quinine, but now quinine is considered unsafe for some people.

See a neurologist for the proper testing, diagnosis, and treatments. One recommended by your regular physician would be best.