If you've already been doing "tons of cardio", then you should definitely start lifting ASAP. Even a basic program three or four days a week will help to build muscle and improve your shape further.
Lots of different programs on the site here could fit the bill. Any of these would be a good place to start:
Check that link Strumpet posted. It sounds like that's what you're dealing with, but your doctor should be able to give you some better input. It could be as simple as just avoiding direct ab work (no crunch, planks, or whatever) and minimizing ab tension (no breath holding when lifting) or it could need surgical repair.