Airtel — Prepaid connection converted to Postpaid

I purchased a prepaid plan for a new connection. Got it for 3 months for 2800 and it got expired.
Now, the company is charging me for a postpaid service that I never signed up for, they are asking me to pay monthly bill however my connection is prepaid.
I never used that connection since March and now I am getting charged for the service I am not even using.
I Purchased the new connection on 3rd Jan and it got expired on 5th April.
Now the bill is generated for 13th May.