incorrect billing

We received our Feb statement on the 12th Feb 2016 for an amount of R2800. Then another statement on the 23rd Feb 2016 R10188-37. On calling I was informed it was infact R10188-37 and asking as how this was possible I was mumbled something about crediting and allocations. We pay our bills like clock work . My husband is unemployed I work as a cashier. We have to on a few days notice come up with an amount that absolutely cannot be correct. I have to take a days leave tmrw to sort it out. How is this even possible 2 statements totally different . It seems they can do just what they like and they don't care at all