Sometimes, if you have a router that doesn't reuse dynamic IPs locally, your local IP address will change then your port forwarding will redirect to a local IP that no longer exists. (like your local IP used to be 192.168.1.100 and it's now 192.168.1.101)
Another reason it could happen would be
http://en.wikipedia.org/wiki/Traffic_shapingIf a reboot works (reboot, it all, restart modem and router try and upload again and see if it goes the same rate) then it could be the amount of connections you have open where Windows could be maxed out (then it would usually show in the Event Viewer under Application Errors) reduce max connections in your Torrent app to 200 or lower. Or could be a older router with out of date firmware that has weak uPNP that floods the router and traffic with error messages and slows things down, but usually after a restarting the router and it could be good for a few hours or a few days and there are usually errors in the router log if you have the option turned on.
Could be the Torrent application you used updated. Some auto update, it may have turned on a setting like Encryption requiring Full Stream or changed the port or ports that are being forwarded (sometimes they appear correct, but in the ini file or in the registry, they are different, could change them, save and change them back)
Could be something else plugged your network (new wireless printer, another computer or something) doubtful, but if it's spewing out connection errors or downloading or loaded with malware it could slow everything down.
Or it simply could be the people that are downloading off of you.
If you run a test and it looks normal or the same, then it's probably nothing.
http://www.speedtest.net
