Has your dog been rubbing its rear end on the ground or licking at herself more so lately?
What is most likely going on with your dog is fluid collection in her anal glands. It has a distinct fishy smell. The glands are just inside the anus in the walls of the rectum. The fluid should be expressed each time your dog passes stool. This keeps the fluid from building up. If the fluid cannot express easily your dog may try to rub or scoot their hind end on the ground to help express the fluid.
If this doesn't help express the fluid then it will have to be done for her. Many groomers and all veterinarians will do this for your dog.
If the fluid builds up enough the anal glands can become infected and/or rupture. So this is not a dire emergency but the sooner you can get her relief the better.
There is no exact reason some dogs/cats have more fluid build up than others or why they can't express their own glands. There have been no good long term treatments other than removing the anal glands surgically.
If I can answer anything else please let me know.
The smell will get better!