Bad throttling policy

Last month I were still a premium uncapped adsl user because I had other people living with me and they are also gamers but this month it's only me and my parents so my parents don't use a lot of data so I switched back to normal 6mbps uncapped line from this month but now I am already throttled for last month's usage which I feel is unfair because they say they throttle you for your last 30 days usage but how can they throttle you for data that you spent while you were a premium user the previous month when you had 600GB fair usage and now I'm on a normal line so my fair usage is 150GB. I have only used 26GB so far this month while being on the normal line. Please respond quickly because I seriously feel like cancelling my contract and spreading the word of this bad service