Hi Kristina.
You name is very beautiful and it is becoming more and more popular in Russia. And mostly after we've got this famous singer Kristina Orbacaite who is a symbol of elegance among Russian teens. I think the reason why your grandparents called you Kristya was that it's very common in Russia to give babies and children diminutive names by adding the suff (Kristinochka, Kristulenka, Natik, Natulik, etc.). But you are a young lady now and it's more approppriate to be called Kristina, let alone that the name is very mysteriuos by itself and it definitely derived from the name of Jesus Crist. I'm sure that the name Kristya will sound funny and not attractive for Russian teens as well as Tasha. If you want to socialize in Russia use your real name Kristina and Natasha, believe me it's cool.

Svetlana (former Svetulya).