Sounds like diastis recti....separation of the abdominal muscles....see some videos on YouTube specifically for this. Regular ab workouts can make this worse, so check out some pre and post natal PTs. I think there might be some apps for this too. Good luck!
Reformer pilates is amazing for toning (including tummy) - my fave exercise xx
Agree with Scarlett - if it’s weight you could always check your macros, I really recommend Lifesum to track your food and macros! If it’s toning up you need then I recommend one of the waist trainer type bands you can get (the workout Velcro type), they help with toning up that area specifically. Good luck with it all and well done for smashing the gym!