Could be pneumonia. Keep a close eye on her and if she seems to be ill in herself, get the vet, they can go downhill quickly. General good 'nursing', ie no stress, nice things to eat will help, willow branches are good.
Have you got a mineral supp, has she had some recently?
If she doesn't pick up with the tetracycline in a day, pen and strep is a good antibiotic for pneumonia.