unusual write off of 3g internet balance

Location/place: vadodara

Name of company/service: tata docomo

i have been using tata dococmo card for my 3g dongle since about 3 months. in the very starting everything worked well but for some recent weeks i have been experiencing unusual write off of my 3g internet balance the pack costing rs.90 which provides 300 mb + 300 mb promotional datapack which earlier used to last long was eroded in as less as 3 days on an average use of 2 hours per day for purely surfing and minor downloads. please help me in this matter.