Prepaid balance reduced with active Tariff

Hello,

I had a balance of about 9-10 euros, and I had recharged 10 euros on 1st July. My pack of 15 euros got reactivated. I checked after that, and had a balance of about 4-5 euros. However, when I check now, after about 20 days, the balance has dropped down to 0.62 euros, even while the tarifplan with EU flat-rate calls and 15 GB data is still active and under the limit.

Please help me to rectify this issue urgently, as my tariff is nearing its expiration.

Thanks