Airtel Prepaid
Without my consent balance is being deducted

I am holding prepaid connection of airtel (mobile number-9970978609) my prblm is that my balance is automatically deducted for no reasons i dont hv any vlue added service activated on my no. but still whenever i rechrge my acc my balance gets deducted! yesterday i got a recharge of 30rs i.e.24min talktime but today its rs18! btw i didn even cal sum 1 from 2 days and em 9th class student so i dont hv much money also! Please understand my problem and try to solve this as soon as possible.hoping for immediate actions.

Thanks