Unhappy over bill!!

I have never been so unhappy with a phone bill. My bill is suppose to be normally R761 per month but yet every month I end up paying a R1000 or more.We called vodacom to cap the amount but I rather receive messages saying that I have exceeded my cap limit, JUST SHOCKING !! . Can this be fixed or not ? On the 22 of May 2015 my bill was R1173 and on the 26 May 2015 the bill is R 1346. Really don't understand how it just increases so quickly.