Internet charged double

I took out a contract some years ago with MTN for INTERNET. I actually forgot to upgrade it, in fact I didn't even think about it needed upgrading. I was never told to upgrade. A month ago they called me and told me I can upgrade. I told the lady I will cannot talk to her that day and will try sort it out. She offered me 2x options but I didn't know which one to accept and told her I will speak to my son. I made it CLEAR that I cannot afford to pay ONE CENT MORE on my internet bill, and would, as with my and my sons phones, have a SET LIMIT. She never contacted me again and I became quite busy and forgot about the upgrade. I received my account last week and, it has DOUBLED!!! Now what did she NOT understand? 1) NEVER called me again 2) I told her NO INCREASE in fees please!!! I am fuming - who is to pay the bill??? I am SICK of MTN and all the tricks everyone pulls. I work long hours and why didn't MTN send me an email or letter saying I need to upgrade. They don't have the deal I am on anymore, I SIGNED FOR THAT DEAL and I understand I was contacted but she did say she will give us time and call again. Please can they trace the call and listen to the recording.