The Op's issue sounds similar to something I have had. Mine is on the back of the leg, behind the knee but just to the side, extending towards the front. It does not bother me whilst running. A few hours later it feels quite uncomfortable when walking down stairs, from the bending of the leg with my body weight. And my prmc is in 5 days...
I did have a cold several days ago and I do often get sensitive tissue with these things, old injuries play up a little, it happens to be on the leg where I had a rugby injury in school. It may be nothing to worry about.