Billed me for data usage I am not subscribed to

Hi I have had a modem (2GB data limit) and a laptop from Nashua. My contract is ending this month and in NOvember last year 2014, Nashua switched over to Altech. I have been billed this month for the usage of 5,5 GB of data when my contract is only for 2 GB. They have taken the amount of R3600 out of my bank account instead of the constant amount of R314, which I signed for. It has been a hassle and an emotional draining situation since last Saturday (28-2-2015) trying to figure out why this has happened. This is a huge amount of money out of my account and no one can tell me why at Altech because they do not have my contract from Nashua on file. It has caused an inconvenience in my life I had to book of 2 days sick at work because I worry myself sick! They said they will refund me if they get hold of the contract as I do not have a copy. This is a matter of urgency and I would like to resolve this asap. I am sure you understand why I am filing this complaint. It is not towards Bianca Shozi but towards the large amount of money deducted from my bank account which I have not signed for. Thus in short I need to locate my contract from Nashua asap!!! Kind regards Deshree Munien