I agree with Karla (Misfits4Me) it could well be stump pyometra.
Or....it is infection caused by her surgery.
A diagnosis of "Systemic Infection" and a month's worth of oral antibiotics isn't enough.
That means an infection which has affected her whole system. It's right, she probably needs IV antibiotics and IV fluids, as her kidneys will be under terrific strain.
She needs fast emergency care, but with a vet who knows what they are doing. I don't know if you have the time, but I would advise getting a second opinion from a really good vet, if anyone where you live could recommend a vet they trust?
Welcome.....You may have an Emergency here.....If this is "Stump Pyometra" from the spay, it is VERY dangerous......You need to call your Vet immediately or find another Vet that will see your girl.....I would go with a new Vet!
Oral Antibiotics may not be enough.......She probably needs IV antibiotics.......Please, make the call ASAP......This infection must be treated with full force, or she won't make it....
Let me know what you find out.....Good luck & I hope it all turns out......Karla
She might be in pain, and it might take awhile after the medicines are out of her system for her to start to feel better. You might want to call your vet and see what they say.