|The pain getting worse hints on the plate being loose. If that's the case, you need some surgery in any case. And there is no sense in tightening the screws when you're cut up again. Then they can also remove the thing and thereby get rid of the cause of your pain.
I reckon that you shouldn't need a cast as heavy as the previous one. There's nothing broken this time, isn't it? Did they tell you how the cast will be? I wonder why you should need one at all. But it's better to follow the doctor's instructions.