Cell c is *****ing my airtime

I've loaded a whatsapp bundle 3 times in a week! That is 1800mb just on whatsapp! That is impossible because my phone is usually on flight mode and only use whatsapp actively after 6. The time on whatsapp during the day is very minimal because I am at school! Please help me get my airtime reimbursedo. Cell c keeps telling me that I used that much when I haven't,. This bundle used to last the whole month and now it runs out in a day. This is a ******* please help. R15 is a lot