Family pension suddenly reduced - How to find out the reasons - Please help approach steps

Sir, Madam,
I am getting family pension on behalf off my husband death while in service; This is happending for last 25+ years. Starting from Jan 2016 suddenly they are posting Rs. 3500/- less than what I am getting in 2015 year.
I am not able to figure out the reasons when I checked with SBI bank. Can you please help how to find out the reasons, where to send my request, whom I need to approach for finding the reasons for sudden reduction of pension. Your help is highly appreciated, please guide me as soon as possible. Thank you.

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