Missing Data

I recently went onto Vodacom pay as you go. Mid January I bought 500Mb of data valid for 30 days, expiring on the 15th of February. On the 9th of February I bought an additional 150Mb also valid for 30days. On the 15th of Feb I had a total of approximately 188Mb left - so logically 38Mb of this would be from the original 500Mb purchased and the remaining 150Mb from the new bundle purchased on the 9th, however at midnight on the 15th ALL of my data was cleared including the new 150Mb! When I called and explained this to the call centre I was told that I must have used all the data without realising which is impossible as I am connected to WiFi at work and at home and all my updates are set to happen when connected to WiFi only. Since purchasing the 500Mb bundle I have not used more than approximately 10Mb per day as indicated by my Data Traffic Management. This DTM also indicates that I only used approximately 6Mb of data between the 15th and 16th, so can you please explain where the rest went?