Account shoots up overnight

I have been with MTN for the last 13 or 14 years with the same number and all. Over the last few months i noticed my account is going from an average of R1000 a month to over R5000. I went to Tygervalley to see if i can sort it out and they claimed it was out of bundle data usage. They did log a complaint and informed me they will refund me R1300. I work outside of SA atleast 2 to 3 weeks in a month and i do not have roaming on my phone. How does this explain the bill of over R5000 and previous months bills of between R2500 and R5500? My Bundle gives me 2 gig data already how much does one need to use to make up a bill of R5000 and this in 1 to 2 weeks. My phone show usage of 2,4 to 2,7 gig and they claim i used over 6 gigs. Please sort this out or i will take it further. Theating a customer like this after 13 years what a shock.