Idea cellular is cheating me.

5-6 months ago (April) .I converted my idea post paid card into prepaid card.And I paid all the previous charges .My card got converted to prepaid.And now after 4 Months Idea send me the message that “Your idea postpaid bill of amount Rs 598 is pending”My question is if I have already cleared all the payments then for what they are charging me.I had done all these conversion process in idea gallery centre and now they are forcing me to pay this amount.Which is totally wrong.