Joyful, wow, I don't have pain like you are describing, thankfully! I'm glad it seems you figured it out. Mine is weird and I'm pretty sure it's a ligament because I can almost feel the fibers attach from the knee cap around the right side of my leg and back to the back of the knee. I feel it twinge, like it wants to snap. It doesn't hurt as much as it scares me.
I don't want it to snap! LOL! It does cause a bit of pain though.
It might be the incline. It's the only addition to my routine that's different. That's a real shame because I get a great workout with the incline. It's possible I'm overdoing it, but I'm finally getting results.