Yeah on retrospect; she's definitely sucking her belly in. And standing straighter.

She's also wearing her panties higher, so they're not rolling and stretching under her belly, looking flatter.

There's also some lens distortion making her look skinnier, photo on the left was taken by someone else while the one on the right looks like a selfie, all selfies tend to have slimming lens distortion because they're taken so close.

Might be wishful thinking but I think she hasn't actually lost any weight.

She probably just did this to get fat-hating trolls off her back, because she often just hits back at them but they keep attacking her, focusing on her increasing size. Especially after she took that photo with the plus-size mannequin that was modelled after her body, they kept saying she was promoting an unhealthy lifestyle so she's probably doing this to look healthier and shut them up.