I think a couple of the guys here mentioned that using Under Armour (brand) sportwear under their regular clothes helps keep things together. UA is basically this breathable spandex substance and it formfits over your body, it can seem uncomfortable at first but it's super stretchy, it's firm but not tight enough to cut off circulation. I think what's happening is that since you're exercising and the underlying support of the fat around your belly is being lost, the remaining fat that is attached to your inner skinwall is offsetting your center of balance/gravity... thus making your back uses its muscles to keep you balanced and using your spine as the pivot point... not very fun. The Under Armour will help gather that up and pull it back inside. I read on another post that it was mentioned to possibly go one size smaller which will add additional support and also allow that one purchase to last longer as you continue your weight loss. Under Armour is sold at most sport/fitness stores like Sport Authority but you can also buy it online.
www.underarmour.com is their website. They make gear for larger size not just pencil thin triathletes... I've seen stuff up to 3XL. Your profile said you fit into a 3XL and sometime 2XL. So they should have stuff in your size but I've noticed the stores don't carry large sizes in the best selection and buying online might be a better option. Just a suggestion. Alternatively, your WLS institute probably has a physical therapist on staff or gave you a consult... they can give you some suggestions.