It's possible that she has vaginitis which is causing the itching, and the antibiotics and prednisolone only worked for so long, or she could have a yeast infection which needs an anti-fungal medication, so, the antibiotics didn't do the trick.
The following webpage examines vaginal discharge and also discusses unspayed dogs, but in addition it does contain some information relevant to her symptoms, so I thought it might be helpful:
I'd have her re-evaluated at this time and try to get to the bottom of the problem. Is she urinating normally? Does she ever get UTI's? If she's prone to urinary infections, this might be a related symptom. I realize her last urinalysis was fine, but you can never tell what's happening at this time, a month later. She may be constantly irritated by her own urine, so try cleaning her with something like MalAcetic Wet Wipes/Dry Bath in that area. Look for it, or something similar, in pet supply stores.
It's also possible that something she's coming in contact with when she urinates/defecates or just sits, is causing an allergic reaction.
I hope you and your vet will be able to alleviate her discomfort very soon and I wish you good luck!