MTN contract convert to pay as you go and +-500% increase in monthly cost

I initially bought a 1GB plan for R110, the contract was coming to an end and I started receiving these e-mails from you which stated, I don’t need to do anything: • You will not pay more than you are currently paying • Your contract/s will remain on month-to-month If you look at the statement, you are now charging me R635.18. I never agreed to this, i never signed a contract to say i will pay R635, i went on your mail that you send, saying i will not pay more?!