That's how CDPs work. They can be renegotiated at any time. Unless your reservation says at the bottom "Rate is guaranteed." That's why it's important to know what codes you are using and what they are good for. I'm surprised they sent you an email and told you. Most of the time when I have this issue I only find it when I'm double checking my reservations.
Advice is the same as it always is. Book through Autoslash. They guarantee the rates they offer you, even if Hertz raised it, Autoslash would make it right. Then have them monitor the reservation for price drops.