Take her to the vet. They can do a stool test to determine what type, if any, parasites are present, and they can do blood work if no other external symptoms present for a diagnosis.
I hope you don't have one of those outrageously-priced vets that just do every test in the book in order to hustle you for money.
If you had time, I could mail you a Drontal pill, which is a broad-spectrum wormer and COSTS only a few dollars a pop, but vets charge anywhere from $10-$25 a pop, and that's not including the office visit, check-up, and testing.
It is quite possible that there is something else going on, and of course I am not a vet. Usually, when worms are present, and by the time they have lost weight as you describe, their abdomen also becomes distended, and they look "fat" except for the fact that their bones stick out. This would not happen suddenly, but rather gradually over a period of 2 or 3 months.
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"I have learned that pleasing everyone is impossible, but pissing everyone off is easy and fun as hell":eatu