That has to be gas trapped in there--although what's causing it is a mystery. I assume you've tried OTC anti-gas pills already. Ask your doctor if there's not a prescription anti-gas medicine that will help you. Or can you double or triple up on the OTC stuff? You need some serious help.
You menitoned that you are constipated a lot. Maybe the gas that should be going on through your system is blocked by impactions and so backs up in your stomach? Ask a doctor about going on a laxative program to get yourself moving again and see if that doesn't let the gas out. I'm thinking the phospho soda or go-lyghtly that they give people prior to colonoscopies (or does the colonic irrigation do the same thing?).
I don't know if lighter measures will work for you, but have you tried aloe vera? You can get it in pills (I've heard it can be nasty-thick in liquid form) and works as a laxative. If it's okay with your doctor for long-term use, you might try that once you get cleaned out to keep you from getting backed up again.
I hope you get to feeling better and manage to be in your sister's wedding. You might want to make sure she knows what's going on and how much you are swelling up so she can have a back-up plan in case you can't get into your dress.