Port forwarding depends on the brand of your the router. I'd check out
PortForward.com to double check your configuration.
Also if you goto options -> Speed Guide and it will show you the port uTorrent is trying to use, as well as a button to test it.
As for why, if you're on a router with multiple people the router's DHCP server could have assigned you a different IP than the one you initially used when you setup port forwarding.