Belly fat skin pouch?

The best way to work your core and tone it up is to build muscle.

Instead of just doing situps and crunches, doing a full ab workout once a week, incorporating both body weight and weighted exercise.

A routine might go something like:

Barbell ab roll-out on knees
Mountain Climbers
Ankle Biters
Cocoons**
Weighted situps (on decline bench for good burn)
Air bikes

Usually I try to spend one minute on one exercise, and if I start struggling at around 40 seconds, I'll stay in the range of motion but go slower. Its also great for measuring progress to see if you can do more next time.

(** Don't know if they are actually called cocoons, that's just what I call them. Where you lie flat out on your back with arms at full stretch above your head and then bringing your knees up and arms down; making sure both your lower back/glutes, and shoulders come off the ground.)

When working your core it's best to remember that there are lots of different muscles which contribute to different areas of strength. Such as your siatic muscles down the side, lowers abs and upper abs.

Good luck

Reading your history and looking at your pic, I think that's loose skin. You lost a lot of weight pretty fast so that's what you end up with. It's still better than having the extra weight. Unfortunately, you can't "tone" loose skin. The good news is that you're young enough that it will likely pull back eventually.

my tummys abit like yours at the bottom but Iv had 2 kids within 18months so mine is most likely loose skin. It doesnt look bad from the side but you can see the saggy bit from front view. Im adding ripped in 30 and kettlebell exercises to my routine and my lower tummy has definitely gone down and tightened up abit, try weights and strength exercises if you can