overcharged on electricity

im paying now R800 a month for electricity(previously it was 300-400 a month ) , they are telling there is nothing wrong with their system. wednesday night i topped up with R100 and Friday morning i am informed i am on a minus. how is this possible?, R400 electricy doesnt even take 2 weeks,. i stay alone hardly cook. but they continue to tell me there is nothing wrong with the readings. when you complain all they do is send you a summary they dont care at all. I feel cheated, even during these period they continue to rip us off. so disappointed