Cell c robbing customers of due data

I have a 40 Gig data contract with cell c. 20G daytime and 20G night time. The month of November the decided to rob me of 4Gig on the daytime . They credited me with 4 gig less and now is mid November my daytime data is almost finished. I dont know the reason of robbing me because not anywhere in the contract i was told the data i receive will drop. If this is how u do business after my contract expires i will look for better alternative. My no is ********** 520 .