Airtel Tamilnadu — gprs reduce amount on my account,still i have 50mb free space & 20 days to expire gprs validity

i have airtel prepaid connection on my nokia5800 mobile.i can activated gprs on my mobile.i can subscribe Rs,95% rate cutter also.my rate cutter have 50mb to browse free & validity ends in 31 january.still i have 20 days to go end validity & 50 mb avail browse freely.
but they reduce my amount on my account for the past 5 days.they autometically reduce amount & free space on my account.i have 20 days to validity expire & 50 mb space to browse freely.but why they reduce amount & free space on my account.
i can formate my memory card & hard reset my mobile number of times.but still problem aqquires.
i can contact airtel customer care.but thuey dont know why this happend.plz help me