Worms

I adopted a SPCA cat around the first of January, that had been with the SPCA for three months.  First cat I have owned, and I had it for a month before I realized it had worms, lots of worms.    I treated with Advantage 100 twice in the first month, twice in the second month and once in the third month.  She has gained two pounds her colour has darkened and her last stool sample was negative.   I am still finding worms in her bedding, and sleeping places.  Is this normal?  Shouldn't the worm medicine kill them.  I am beside myself, I thought after the vet told me she was negative there would be no more worms.  If anyone can advice please help me.  

Depending on where you are, it's most likely tapeworms from fleas.  They require a special dewormer.