How are you doing on Quad stretches? FWIW, a tight quad will pull the knee cap to the outside.

Maybe a few sets of squats to strengthen the inside as well as quad stretches.?

"Do you have any ideas of why it would get tight and slighly numb behind the knee cap before the pain starts?"

Could be the beginning of Sciatica caused by a bulging disk in the lower back. Don't know of course and would encourage having it checked out. Disk bulges out, causing numbness and then as it further bulges out, the pain increases in both knee and back.

Could also be an overly tight/developed quad as an overly tight/developed quad will pull the knee cap to the outside with it's associated set of warning problems.

Are you doing hip stretches?

Lay on back, knees brought up.

Pull knee, with both hands, to chest, twice, (don't force as you're just stretching) first time to a count of twenty and the second time to a count of thirty. Both knees, alternating. You need a thirty count to get the tendons to get and hold the stretch message.

Hip stretch two:

Lay on back with knee pulled up, other leg laid out. Take opposite hand to outside of knee and pull the knee over to the side; l-knee to r and second time, r-knee to l. This will stretch the hip tendons.

Do this in an alternating pattern. Pull knee back. Pull second knee back, pull knee over, pull second knee over; 20 count. Do a second time for a thirty count.

Benefit will be noticed fairly quickly over an expected two week time.

FWIW, I suffer from a blown out L3/L4 and this set of stretches keeps me going (loosened up) and has helped many others I've shared it with.

In the simple, for the quad; lay on your tummy and pull your foot back and this will stretch the quad where it attaches just above the knee area. Do this twice, alternating between the l and the r for a twenty count and a thirty count before going out.

Hope this helps.