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!
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
Is it extra weight or loose skin? If it’s weight then you would need to make sure you are in a calorie deficit to loose weight but you cannot spot reduce weight so it would come off from anywhere. If it’s skin then the best you can do is try to tone up. Courtney Black on insta has some free ab workouts on her insta as well as more on her paid app x