Rays can be finicky, and from my understanding retics are one of the more sensitive species
Sucks that you lost her, I would just test your water often until you figure out for sure why it is spiking but I am pretty sure your filter cleaning did it.
Did you use tap water or tank water to clean the filter?
Ray keeping can humble some pretty experienced fish keepers, everything is magnified ten fold with the amount of waste and food that come along with rays.