Hi Spine95 -- is it BEHIND the knee or in front? If it's behind, and since you said you think it's originating from the lower back, it could be sciatica. Sciatica DOES make the knee hurt, behind it, plus it goes on down the leg after awhile. It's REALLY painful, and like you say, it's a deep ache -- it feels like it goes thru and into the bone.

I've had it for many many years.
Whatever it is, I'm so sorry you're hurting so. Gosh, I hope nothing went haywire. I hope it's just a spasm or some weird thing that is going to stop and quickly as it started. Please let us know what happens, will you? I'd really like to know what the MRI says, okay?
Take care & good luck! Hugs, Lee