Dog rapidly gaining weight - help?

My dog is a normally 20lb Shih-Tzu/Sharpei mix. Her name is Maxie, and she's a normally laid back dog. She's super intelligent, more so than most dogs I know, like Border-Collie-that-knows-200-words level. She is very well behaved and sleeps a lot, loves treats and human food but is not a big eater. She got a lot better, health and energy level wise, when I did research and started feeding her better food, which at the time was Blue. I did more research and started feeding her Under The Sun, which has everything dogs need and nothing they don't. Ever since then, her energy levels have gone down, she sleeps more, is less attentive and less obedient when in training sessions, and eats less. I don't know if it's because of the food, or because of her age (she turns 3 in May), or because she got spayed about a year ago, right after I started feeding her Blue, or what. All I know is that she has been in between 18 and 22 lbs her whole life and right now she is in between 28 and 30. I'm really concerned about how this weight is affecting her and I'd like to get it off. She gets two scoops of a half cup of food (Under the Sun) twice a day, but almost never eats both bowls entirely in one day. She gets human food and treats but not enough to make her this fat, but we've stopped giving those until we find a solution to her weight gain.

Mostly I came to ask what the best way to go at this is. She will be walked more, no more treats/human food, and we are considering switching her back to Blue but if not, to another food that will help with weight.

TIA!

Personally, I'd run this by a vet as thyroid problems can often lead to weight gains.

Also, if I thought food was behind the problems, I'd change her back to what she was eating before the problem began to see if maybe the current food is not suited to her

I have discovered that it is, unfortunately, most likely her Thyroid. I will be taking her to a vet soon and will get her all fixed up.