credit limit crossed of IEDA

SIr,
I am anil birla mobile number 8378999617 having idea postpaid connectiopn for last three month.but day before yeaterday i have activated 1GB@125 of 3G connection after that i got message that i have internet plan and they also send me settings for it. noe in the evening i used it by knowing that i have net connection as messages send by idea. and in evening i have crossed credit limit of 600rs. and next day i got message of paying 2305rs and they interrupt services when i called customer care than he told me of paying of 8305 rs. and told me that you have crossed credit limit hence you got so huge bill
I want to ask why have not send mesage at the time of crossing credit limit and interrupt service at that time only?second i dont know credit limit so far even Idea has not send single message related to credit limit how customer come to know about it. they are telling that after 24 hours they interrupt service it means they are getting window of time for earning money. why there is not self generated message for it in Idea.