With the drop in download speed that the speedtest indicated, I'd say that your ISP have picked you up as maybe a heavy downloader, or you have used up your monthly quota and they have now shaped you...
I know if I use up my monthly allowance I don't get charged for the extra downloading but I do get shaped down to 64 kB/s until the month is up...
Have a look at your plan and see what it says, even if it says unlimited there is usually still a limit...
