Synchrony Bank Credit Card Complaint

Synchrony Bank Credit card department,

APR or interest rate New York

I have a credit care with Care Credit - I used for dental work - 2015. It was 1 year deferred interest. I made my payment each month - then in - - I hit a bump and did not make a payment - then got back on track. Well, that bump - made them charge me over $900.00 interest. Now I owe more than I charged. Is that right?