Lava Mobiles — factory reset

Address:

Sealdah, Kolkata, West Bengal

I bought lava z60s it has 3 months old. But i have face a problem with my phone. When my phone is working slowly or it was hang then i restart or power off my phone but after power on or after restart my phone is showing "your android data is corrupt please factory reset your phone".
However i go service center in sealdah for solve my problem. But they charge rs 350 for servicing my phone within the warranty period.
Why lava care charge for servicing within warranty period?? ☹️ this is very illogical and unfaithful.