Kids book about aliens taking a human as a pet

I've had this book in my head for ages and no google search has found it. I read it in the primary school library many years ago so it definitely had nothing sexual in it.

It was a scifi novel where a human girl (I think) was taken as a pet by an alien scientist. He was being kind to her, but was still studying her. He would take her for walks on and off leash and if she was obedient and came when called, he'd give her greater freedom. The most striking thing I remember was that the gravity was heavier on the planet she was taken to, and so she talked about how she felt dragged down and heavy. Also, the scientist and the girl could talk to each other. She cooperated with him.

I thought for a long time it was 'Galax-Arena' by Gillian Rubinstein, but don't remember all the circus stuff, although the wiki synopsis does mention aliens taking human children as pets. I am pretty sure the book was very near the Rubinstein ones, so it might have been, but wanted to know if it rang any bells for anyone.

Thanks anyway!