Nine Minute Intervals - Sloppy Knees?

Tonight my knees are kind of bugging me, they're sloppy.  Kind of feel like they may fall out of place at any moment.  I looked this up and found out there is such a thing as "loose knees" or joints.  I came home from the gym and rested for a few hours before I got on the treadmill.  It was incline night.  Time to tackle the nine minute interval.  I made it 54 minutes.  My knees felt like they were going to fall apart.  When I went to an even treadmill for the last six minutes, it felt a lot less uncomfortable.  Now my knees feel like they are wrapped by giant rubber band.  Super tight and stiff.  Got to look up what might be going on here.  Starting to wonder if I might be overtraining a little.  Just a little.