Depletion of data without reason.

This morning at 11:32 I've received a message from Vodacom that I have less than 99MB left. Right after this message I browsed for some recipes about 5MB worth of data, and after I've hardly touched my phone. At 16:37 I've received a message that I have less than 25MB left, and at 16:39 I've received 2 messages, the first of which that said I have 10MB left, and then 0MB left. How is this possible!? Please fix this!