Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
When a dog is in heat, they do not typically run a fever or lose their appetite. Those signs make me worry that she may actually have a pyometra (infection in the uterus) vs a heat cycle
If this is a pyometra, these dogs get very sick quite rapidly. They will have a discharge from the vulva. It can be white, yellow or bloody. They will also often times run a fever and have increased thirst/ urination. As it progresses, they can lose their appetite, vomit and eventually become septic and die. This can all happen in a matter of a few days.
I would get her evaluated by a vet and get either x-rays or ultrasound to rule out pyometra. As long as it's ruled out, some anti-inflammatory meds may help make her more comfortable.
Here is a nice write up on Pyometra in dogs to read over as well:
I hope this helps, let me know if you have any other questions.