Overcharged Account

I was overcharged on my account. I paid R200 in October and the balance was R156. In November when I went to pay the account the balance was R400 some-change. I do not understand how could my balance increase so much and I didn’t even use the account. I was really disappointed when I went to the store and I couldn’t be helped. I always make sure I pay my account in time and above the minimum amount i need to pay.