complaint

i have a telkom wifi R100 -2GB ,for the passed 3 months i have been paying a fixed R100 as per our aggrement.I revievd an invoice for May and guess what it shows i have to pay R600.I phoned telkom and iw as told someone changed my contract ,how pathetic is this reponse i am recieving form this company,they will investigate.How can a contract be changed without me knowing.Apparently they will investigate and get back to me.I have asked them specifically when i signed the contract that i dont want it to exceed the R100,Which the consultant assured me that it has been done(set),since January i have been paying R100 as agreed now June its R600,52.I want this sorted out urgently otherwise i will cancell my contract