Airtel postpaid sim
Wrong bill genration

I m using the airtel postpaid sim since 4 month last 3 month i hav paid maximum 1200 rs. payment but last month company generate my bill 6200 rs. and asking me that my cradit limit was 1200 rs. i m telling him that if my cradit limite was 1200? then why r u waiting the 6200 amount . so pls clearing my this problem