Data charges incorrectly billed

I have a 1.5GB re-occurring data bundle on my line and I check the data balance via *141# on a daily basis. As at the 5th of March I had in excess of 1GB remaining. My bill run is on the 7th. When I got my invoice yesterday I saw PSD calls charged in excess of R480. I would like to know how this is possible & why, especially since I loads of bundle data left between 1-8 March?? This is urgent, thanks. The reason I loaded the bundle was to REDUCE the monthly data expenditure and benefit from the low rate per MB. The previous invoice (February's) PSD calls charges were R2 368.05?! I loaded the data bundle on 23 January 2015, but I suspect there is a billing error here and would appreciate your investigating this as a matter of urgency. Thanks