i had some knee pain on my left leg that i couldn't explain for the longest time (right above the cap). like you i don't fall too much anymore (at least not on my knees) so i didn't quite figure it out. then one day on a chair lift i noticed the pain flaring up. i looked down and realized that it's because my board's dangling from my leg and therefore pulling on it. once i rested it on the bar or held it over the back on the chair it went away, and the pain slowly dissipated.
just an idea. if it's soreness or anything similar you just need to sack up a bit.