I would find a good chiropractor and see them... If it's a pain that is worry you enough to be posting about, and it doesn't go away for a couple of weeks, you should get it checked out.. Any back/torso injuries are worth getting looked at.. They may need to take an x-ray or something.

Alternatively you could see a physio. Try to find one that has specialized in sports and they should be able to give you exercises to help it recover..
It's a strange one to get from throwing punches..

Just rest it a little I guess.. If it doesn't get better, or seems to happen every now and then when your doing that specific movement, then i'd say it's definetly a problem in your back that is off balance, down your spine between your mid back and tailbone,,, and any twisting of that area in the wrong way could give you pains just about anywhere.. Hips particularly... A few cracks of the back and a bit of manipulation and you'll be ready for anything