I had taken Plan 250 BSNL Broadband connection in Sep'2005.For the first two months I got internet bill for 250 only. But for this month i got a bill to pay Rs 356. And one thing, that month i used net very frequently.So, my doubt is eventhough if do not downloaded anything i exceeded my limit. What would be the correct reason for this?

In broadband connection it is different from dialup connection. Here frequency doesnot matter as it is always on conncetion. What matters is the size of the pages you visit. If you visit sites which are having small graphics and more text then you are downloading very small amount of data. But if you are visitng sites with large graphics or streaming videos then you are downloading higher amount of data.