First day using Kindle Unlimited and I know I have a borrow limit of 8 books. I borrowed 6 books and there were 6 books in my content library at first, it looked all good. But I checked again after a few hours and there are more books now, which marked "Prime" instead of "Kindle Unlimited" and they are available for return as well. Am I over the limit? Do I need to return those "Prime" books?