Unwarranted money coming off my account

I opened a cheque account about 5 years ago, with a rebate option, that if i kept a mimimum amount in my account, then i could have unlimited transactions and swipes for free. A few months ago all of a sudden additional money between R200 and R300 started coming off my account, and when i query it, i get no help. \It's probably because of the transactions you're doing". But why all of a sudden? you can't just change fee structures"