There is so much I don't know, and I'm sorry to say that I think Mollie needs to see a vet, particularly with her belly being so swollen. It could be weight gain, but I'd be worried that it's more.
She may have food intolerances, so you might want to try limited ingredient diets to rule out some food for allergies. I give Katie probiotics, and I definitely would check them out. I use Probiotic Max by Nature's Farmacy. I gave them to my friend, and her dog who had many problems, is doing great. Read the links I am sending you. Ann (Wylie's mom), our wonderful moderator, suggested this brand to me, and I really trust their products. I've ordered a number of things from them. Katie doesn't have problems with yeast or any skin problems, but she loves to run and fetch her balls when playing. She will go deep into the shrubs to retrieve her ball if need be. Though I always clean them when she brings it to me, they first are carried in her mouth. I worried about all that dirt upsetting her stomach. I was using another brand made by the company that makes Cosequin, but this is so much better. I would definitely give it a try.
I'm so glad that Mollie was able to be helped with her mammary tumor. She might also be gaining weight because she got spayed when she was older. Katie was three when she was spayed. She gets so much exercise, but it's a struggle to keep her thin. I'm trying so hard to get her to lose weight, and it's a struggle. The weight is not good for their knees or their heart, so I will do what needs to be done to get her to her pre-spay weight.
Welcome to YT. I've had Yorkies for thirty years, and I still learn things new every day.
THE FORGOTTEN ORGAN IN OUR BODIES??? DO YOU KNOW WHAT IT IS? | Natures Farmacy Inc. Systemic Yeast Infections in Dogs | Natures Farmacy Inc. DOGZYMES® Probiotic Max – 8 Ounces » Natures Farmacy Inc.