Short term loan debiting money randomly

Took a short term loan with Fnb, they keep deducting random amounts, today worse they deducted all the balance I had in the account leaving me in very highly inconvenienced. I mean how can they take money a they please even the agreed payment date. I mean if I take a short term must be given an opportunity to pay on agreed not randomly deduct amounts without warning. This sucks from claiming to but *****ing customers over.