8 gig in 3 hours??????

I have two Mtn contracts for numerous years. On the 22 st of December I received an sms stating my bill on the one account amounts to R8000 for data usage ( which is supposed to have a limit) due to the fact than I am a pensioner. Upon investigation at Mtn Village Mall Hartebee****rtdam I was informed that data to the value of this amount (8gig) was used in a matter of 3 hours on the 21 st December. During that time on the 21 st December when all the data was used on my phone i was not using my phone. My phone was on charge at home while I went shopping. No software or apps showed that they were updated on my phone. Available network coverage at my house is less than 14.4 Mbps. How is it possible to download 8 gig in three hours???? If it is 14.4 per minute times three hours then it equals to less than 3 gig and defintly not 8 gig!!! This was the worst festive seasons ever because of this!!!!!!