Cortisol can cause this

Dieting is a stress to the body. It will release Cortisol in response to this.

Wheat products can cause this.

Artificial sweeteners
Dairy

Also there is something about apparently fat Celsius after a cut are filled with water and then you peebitbout and you get that "swoosh".l effect of weight loss

So look at diet stress reduction and give it time it will normalised unless oestrogen is high