My platties were producing white poop last week, probably caused by the oil my son put in the tank but they've recovered just fine. Seemed it was just a tummy upset (The oil was 100% natural apricot). It could be the guppy's digestive system isn't terribly impressed with the medication and it has a bit of an upset tummy. Or it could be something else. Just keep your eyes on her. If she seems unwell it may be she's reacting to the meds or just feels stressed.
Hugs,
P.