Data Usage Shock

After receiving my bill I noticed that my data usage for this month was abnormally large ( R2047 ). On my itemised billing it shows that on the 24/03/2016 I used around R1500 worth out of bundle data but then the very next day 25/03/2016 it shows i was back to using in bundle rates (This surely isn't possible). I called customer care and queried this to which the response was that this is normal and will happen if what you streaming/downloading etc is bigger than the remaining bundle. This makes absolutely no sense to me because surely if an item is bigger than your remaining bundle it will use the whole bundle then go into out of bundle rates NOT use out of bundle rates then go back into bundle rates the next day.. On top of this its really odd that on this day the 24/03/2015 my data usage is at least 3GB MORE than any other day.. PLEASE PLEASE PLEASE HELP !!!