@Nancy829
You have 20% special discount for first 3 bookings and a 10% monthly discount set on your listing.
It calculates normally in May and June, but indeed in July (and August etc..) something is wrong. I think by accident you have set a fixed price per month, so check your pricing system under "Length-of-stay discounts", option: "You can also set a custom price for a specific week or month" and click on "month"
If you can not solve this problem very soon, i suggest to set your maximum stay temporary to 27 nights, so a month's stay can not be booked anyway.
Hope this helps.