Check your shampoo bottle label.
I don't know WHY I didn’t figure this out sooner. It’s the shampoo I use in the shower!
When I wash my hair, the shampoo runs down my whole body and (duh!) printed very clearly on the shampoo label is this warning,
*FOR EXTRA VOLUME AND BODY!*
NO wonder I have been gaining weight!!!
Well! I have gotten rid of that shampoo and I am going to start using Dawn dish soap instead.
Their label reads,
*DISSOLVES FAT THAT IS OTHERWISE DIFFICULT TO REMOVE*.