Hmm, well it sounds like you're confusing uppercut with a body hook or shovel punch. You shouldn't really be able to hit the bag with an uppercut, since it comes straight up. When throwing an uppercut, you should practically skim off of your opponent chest before hitting their chin.
A shovel punch resembles a body hook, but it has your palm facing up, so you "dig" into that floating rib. Just keep that wrist straight and in line.
Tommy Hearns was 6'0+ fighting at 135, I believe. If your body is capable of dropping that low, do it! Yoshihiro Sato, I BELIEVE used to fight at 132. But I don't recommend - you'll have hardly any muscle mass! Look like Voldo from Soul Calibur
If it helps, remove some of the stuffing inside of the bag -- it may be too solid. Then work your way back up. Are you punching correctly? You hand isn't bending backwards when you land is it? And it's mostly your knuckles that are hitting the bag? Ice your wrists if they are hurting, before you get back into the rhythm of things.
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