Hi Jimmy_T,
Have a look at both of the stickies above.
Assuming you haven't changed any of your settings (eg % of bandwidth devoted to downloads), it could be an issue with your ISP.
There's a thread within this section titled "Increasing download speed?", have a read of it. It gives you a link to a speed test.
Also, try using the search tool (top right between faq and home). Maybe search for 'decrease in download speed' or something similar.
Good luck!