What are the causes for female shelties weight gain 7months after having puppies?

My female had a litter back in october that she nursed for 3 months.Some of her nipples and belly still have black spots.While nursing she stayed thin but since then she has tripled in weight.She doesnt eat very much and is not sick.I have pressed on her belly and all over to see if she is tender somewhere and she acts fine.She eats,plays and potties normally but is just getting bigger and bigger.Her food is the same as before she was pregnant. When I called our vet he said if she is eating and relieving her self and acting normal there isnt any thing to worry about.We are considering switching vets because we think something is wrong.Any help out there will be appreciated.We are going to call someone after the holiday .

If your positive that she hasn't been getting into the garbage, table scraps, cat food, and there is no other change in her diet go to a different vet. Dogs can get hypothyroidism which can make them gain weight even when there hasn't been a change in their diet. . Good Luck