Overcharging for data charges

Altech seem to be overcharging for data usage. I rarely go onto the internet but to check my mail. I used to have a 500MB limit and never once reached my cap. I then was upgraded to a 1GB limit and also never reached my cap. I have now been upgraded to a 2GB cap and have exceeded my limit on 3 occasions being billed once in excess of R2000 for supposedly using over 10GB's. I then compared the MB's when I plugged in my modem with Altech to my work modem which is directly through Vodacom both just having my yahoo mail open going through mail. In a 10 minute period with Altech I used over 100MB's. With my Vodacom modem I used just 15MB's. My wife has the same contract with Altech with a 2MB cap. She recently went in to close her account and they said her last months bill is R18 000 and my wife said she had never used her modem. She predominantly uses her prepaid Ipad. Please can you investigate this