RB951Ui Port 3 LED is ON all the time, even when no cable is plugged there

Hello,
I have an issue with my RB, as the title says it all, once i power on the router, the LED for port 3 turns on, even withough connecting anything, plus i could no longer access it using winbox, even after a hard reset, I had to reload the firmware using this (http://forum.mikrotik.com/t/new-rb951g-2hnd-bricked-solved-with-netinstall/70728/1) method, and now i have access to the router using winbox, but still the port 3 remains on.

What could cause that ? have you ever experienced this problem ?
How can i fix it ? and avoid it in the future?

Thanks!