Vodacom take your data without you knowing

I have a 2 GB Wifi router. In March i depleted my data and loaded 9 GB of data as i was downloading a lot. In April my new data kicked in and i checked my account on the 17/04/2016. It showed that i had 7.6 GB of data that was expiring today and nothing left on my data bundle. I contacted the call center and was told that at the beginning of the month when you new data starts that gets depleted first instead of the data that you purchased which is only valid for 30 days. This is so wrong. It is like they are *****ing you data from you... Well i asked to speak to a manager and after waiting for 10 minutes gave up and put the phone down.