As far as I know, henna usually improves ponytail circumference because it coats the hair making it coarser. I have never experienced hair loss after henna. I had a little shedding when I rinsed henna, but not after.
Have you changed anything in your routine? Maybe you should eat healthier, or take vitamins?
Maybe you just pull some hair while you henna. Sometimes, expecially when the mud is solidifying on my head, I feel some hair pulling, maybe this is the case, expecially since you said your mother had hair loss with cassia. If she had it with cassia, and you had it with henna, maybe it is something physical more than chemical.