that's because you've overworked yourself, you have to go slow at first and take it easy until your muscles can get used to it, basicly, your muscles have tightened up because of the workout and you need to make them relax. Heres what i do when it happends to me.
If you want to straighten it out i suggest doing it very very slowly, do a little and wait til the pain eases off, then do some more, and wait, do a little more, and wait til eventually your arms straight, then try avoid keeping your arm bent for too long or it'll seize up again and you'll have the same problem. Try lieing on it during the night while it's straight to keep it straight. Thats what i do, seems to work after a few days and gives me use of my arm again.
Get some muscle rub, it'll help ease the pain a bit more aswell.