I don't know if I could help or not. It doesn't sound normal for them to put on weight though.

How old is your dog? Are you planning on having her spayed? It does get pretty tiresome if you don't get her spayed. And It is better for their health if you have them spayed.

All my years though I have never see them gain weight, when they were in heat. They stay in heat 21 days. The only time I have seen them gain weight is when they are pregnant.
Any chance of that?

Sorry I couldn't be of more help than that.

Willie