Sounds like you may have previously left a sharp corner when you previously clipped your nail which can catch on your skin as the nail re-grows and lead to an ingrown situation.
Since it is not red or swollen, that sharp edge may just now be starting to "poke" the tissue during the normal flexion of the food while walking.
Here a site you may find useful. But read the last sentence in #8 and then read #13 before starting back at #1 (I don't think you have the typical ingrown toenail issue.)
http://www.wikihow.com/Remove-an-Ingrown-Toenail