Sorry i'm a bit late to this thread, but i've got some thoughts that might clear up some problems like those Buzzbee has. It's a very similar problem to what I sometimes experience, and it's a combination of "too much" cycling and sitting and not enough other exercises.
The position a bike puts you into never fully stretches your hip flexors and two other muscles (the ones that cause me the most problems), the rectus femoris and the sartorius. Both of these muscles are involved in knee extension, but since they cross the hip, during cycling, they are never fully lengthened (since you're bent forward). If you spend enough time sitting (on a bike, chair, car, etc.), they will shorten. When you then perform an exercise the requires them to be maximally lengthened, like during the eccentric contractions involved with downhill walking/running, they end up getting overstretched. This ultimately leads to minor damage and, subsequently, sometimes debilitating cramping. The reason it normally doesn't happen when going uphill is because you're leaning forward a bit and the leg doesn't quite stretch those muscles out all the way. For Buzzbee anyway, I highly doubt that it's anything nerve related or referred pain, because generally that would affect everything downstream of that nerve (toes, foot, calf, etc). Sometimes general physicians aren't used to problems experienced by active people and forget Occam's Razor.
Like an earlier commenter, I've realized that this is more likely to happen to me when i'm traveling. I don't think its so much that i'm not getting my normal workouts, but I theorize that it's because I spend an extended period of time sitting in a car or airplane seat, which further shortens those muscles and makes them more susceptible to being overstretched when I do something more strenuous.
The best way to prevent this from happening, without cutting back on cycling or sitting, is to keep the muscles used to acting through their full range of motion. Stretching obviously will help. Quad stretches, lunge stretch, etc. Also doing things like running and resistance training exercises that work them will help. Good luck everyone!