I'm surprised she has worms and yet the vet didn't find any in her fecal? I know tapes can be harder to identify via a fecal but even so it just strikes me as odd.

Regardless, if you can find one or have a stool sample with some in it, bring it to the vet. You typically don't have to have an appointment to drop off feces for a fecal. If you can't get to the vet soon, you can always put it in the refrigerator (NOT the freezer) to bring to the vet later. Now I know a lot of people aren't comfortable with putting feces in their fridge... but I guess double or triple bag it, put it in an air-proof container or do whatever you feel needs to be done and put it away from the food I suppose