Hello Bruce! Welcome to The Cat Site.
Cats often do this when they are really pleased. I call it jiggly-tail-kitty....one of my cats does this whenever I come home and when she is just about to get a little tuna as a treat.
Also, this is what it looks like when a cat sprays. Have you noticed any urine when she does this?
And...it sure is odd that she only does it when she passes the toilet. Hmmmmmm......