I seriously doubt that your grandmother would be confused. To me that's like saying that I will ever in my lifetime forgot what sorority I'm in. Maybe Pi Tau Delta was just a local sorority? I think you should talk to her about it. She probably still has her pin and other sorority items to show you.
P.S. Pi Tau Delta is the International Chiropractic Honor Society as Teeny said. Since she was pre-med maybe she somehow joined this?
------------------
If the angels up above
Could hear us sing our song to you
They'd see that in our hearts
We're Pi Phi's from the start