Amazon has changed the way it pays for borrowed books, called KENP, Kindle Edition Normalized Page count. Perhaps you've seen the discussion about it in forums, blogs and elsewhere. Basically, it affects the books that are borrowed through Prime and Kindle Unlimited.
Before, when someone borrowed my book, I would get the same amount of money that everyone did, regardless of the length of the book. Though not fair for the higher priced books, I was happy with the result. A a borrow would get about 6-7 times the royalty of a sale.
Now, amazon is trying to make this more fair. They are doing payouts on borrowed books based on the amount of pages read. I don't know yet how much each page is worth, it may be around $ .0058. For my 45 page book, I'll receive an amount quite a bit less less than a sold book.
The system will be better for longer books, but for the shorter ones like mine, it may not be as good. If it continues to be less for me, I'll consider taking it off the borrow list. After all, it's only $ .99. And I might look at Smashwords again.
I'll wait and see a little longer how it goes. In the meantime, I'm hashing out more of my next book.