Moose is just turning 5 months tomorrow.

Junior tried humping her the day we brought her home
And since then no interest till a couple days ago.
Now he is humping her again and of course I am not
Allowing it. But could she becoming into heat the reason
Why he is now doing this? She is not swelling or anything.

What signs should I be watching for? Yes planning
On spaying her, but want to try and let her mature first.