I would have to agree with HereAndThereSC. When you change your reservation, it often gets a reprice. Most of the time it works to my advantage and the discount is greater a few days before the trip.

It's possible that they are now sold out of the car you want on the days you want them. Where previously you were picking up on days where they had an overabundance of cars. There are several locations I've been to where they do not have enough parking spaces if all their cars came back on the same day. They give big discounts on those days where there was that danger, instead of requiring them to pay for offsite parking.