You said you've had her three weeks and haven't seen a hard stool yet so it sounds like it's more than the antibiotics she recently started. Have you had her dewormed? I remember when we brought Jazz home she had runny stools and was on antibiotics so I assumed it was that but turns out she also had tapeworms.

Her stool should be like a tootsie roll. Firm, shaped like a log but not rock hard either.