Airtel — extra charge and received bill even after converting from postpaid to prepaid

Hi.
When i ported me aircel account to airtell account, i had been told that i would have to opt for post paid compulsorily for 4 months. I had taken plan of 199rs. But i had been given bill of 199 in first month and then 240, 280 and 240 rs for the remaining months. After 4 months when i put the request for converting to airtell prepaid, i had been told that i would get a call for confirmation. The company hr called me and i clarified her about the same. Even after that she put the comment that i wanted to remain in post paid plan. After fwe days when i again asked them regarding the same they told that i have opted for post paid only. After that i had to put new request for pre paid account. I cleared all my dues. Now in last month i got pre paid but still i got bill of 41 rs.
I need the clarification for the same.