What boxiana said, plus practice rotating your upper torso and hips into your power punches, power not only comes from the legs.
I've never had that but I do know that any part of your body that suddenly gets overused might have problems with the workload. When I first started hitting the bags my forearms would hurt but after a few months they stopped. Recently I've gotten some work related pains for the same reasons, I'm using the old standby's of rest, ice, elevation and some heat treatments. One thing I know for a fact is, you can't tough your way through every pain, sometimes it's the exact opposite, you just need to stop and rest as inconvenient as that might be. Also, I try to take supplements, like good multivitamins and glucosomine. One of the most important things that I've learned in all my years though is that you have to warm up thoroughly before you start, you have to prime your body for what you're about to throw at it, to not spend a few minutes doing this will cause you lots more wasted time later.
