Not sure if yours is the same, but I used to have a lot of inexplicable pain on the inside of my quad, about 2/3s of the way down. I eventually realized that it was connected to having very tight groin/abs on that side.
Edit: I guess that's the psoas muscle, so you probably didn't need my advice.
Interesting. Well I was completely guessing on the psoas, so it's nice to get some confirmation that someone else's inexplicable quad pain that sounds like it was in about the same spot seems to be from psoas tightness. Regardless, psoas stretches feel awesome. My quad's feeling tons better now, so hopefully things are back to normal!