You could be describing my current predicament.
I've been dealing with runner's knee (patellofemoral syndrome) in my right knee for a few months now. Two weeks ago it was on the cusp of breaking down and finally did last week.
I took a day off, tried to run and realized I couldn't, repeated this sequence the next two days, then resigned myself to a week of no running. My knee is improving but I can tell it'll require at least a few weeks of zero running to heal sufficiently enough that I can be reasonably confident it won't return when I resume.
In the meantime I've been stretching my hamstrings and doing body-weight exercises, like squats, much more frequently than I was. And because my knee is visibly swollen I've been icing it regularly. This seems to help.
I think the recommendation to begin or continue strengthening exercises is a good one.
wbbv, you say you feel no pain. Does this hold true for the entire range over which you can flex your leg? Even with all your weight on that leg?
I ask because my pain is worst when my knee is flexed at a certain angle -- and with my weight of course put entirely on that leg. This is what most hampers my running: I can guard against the pain by running with a short, choppy stride, but of course running fast or uphill leaves my knee unprotected.
Good luck with your injury.