Yes, this does rather sound like Pyometra symptoms. My dog had Pyometra last year.
What's happening is the womb is full of pus. It is swollen to at least twice its normal size, and toxins from that infected womb are slowly spreading into her bloodstream.
The only way I can recommend you deal with this is to get the womb removed -and as soon as you can. Now that is just MY opinion. Some people (especially if they want to breed their dog in the future) opt for treatment with antibiotics, which can help, but usualy doesn't solve the problem once-and-for-all. But Pyometras which are treated medically, rather than surgically, are very much inclined to recur, and can recur very quickly...most likely at the next "season".
It would be probably more expensive, and would certainly be harder for her to cope with, to have this serious problem returning time and again, with continued vet visits, and antibiotic treatments. Then one day, her kidneys wouldn't be able to stand the strain of filtering bad blood regularly, and may pack up entirely
I seriously think the best plan -for her health, and probably financially too -is to have her spayed. And the sooner the better.
Pyometra is deadly serious.