It may be Iliotibial Band Friction Syndrome (IBFS).
Your iliotibial band (ITB) is a thick band of fibers that begins at the most prominent bone in your pelvis and runs on the lateral (outside) part of the the thigh until it attaches into the tibia (shinbone). Your glutes (butt muscles) and hip muscles attach to it. Its job is to coordinate the muscles and stabilize the knee during running.
IBFS results from the band becoming irritated and inflamed as it crosses over the lower, wider portion of the femur (thighbone). The inflammation sometimes results from the generated by the constant bending and straightening of the knee that occurs during the running motion. The most common cause of IBFS, however, is an overly tilted pelvis.
IBFS-causing hip tilt can be caused by both internal and external geometries. Internally, anatomical issues such as bowed legs, differences in leg length, excessive pronation (rolling inward of the foot/ankle), and weak or tight glutes or quad muscles can cause the tilt. The most common external cause is when a an athlete repeatedly runs on the same side of a highly cambered or sloped road.
What you feel on the outside of your knee:
tightness that becomes a burning or stinging sensation when you run
no pain when not running or walking stiff-legged
pain when you stand up after being seated
a snapping, popping, or "giving" sensation
Initial treatment recommendations:
stop training activities that causes the pain, but continue pain-free training activities
apply ice packs to the knee for 15 to 20 minutes, 3 to 4 times a day
wear an IT Band strap when running to reduce IT Band deflection across the femur
Avoid a recurrence by:
warming up slowly and replicating the type of movement you will use during activity
increasing your training intensity gradually, no more than 10% each week
avoiding repeated running on a banked surface
reducing pronation related issues by wearing with proper footwear
incorporating these exercises (videos hosted by Fleet Feet Training Educator, Adam Willis):
Fleet Feet Training Director Brandi Barbre has more than a decade of experience coaching athletic performance, prescribing corrective exercise, and conducting metabolic testing. She has trained thousands of post-collegiate runners over a variety of distances, hundreds of school-aged runners, and dozens of professional athletes (18 NHLers, 11 NFLers, 7 MLBers, and 4 MLSers). Brandi has a Masters in Exercise Science from Southeast Missouri State University, has earned ACSM CPT, NSCA CSCS, and USAW Sports Performance Coach certifications, and is a marathoner (19 fulls; 28 halfs) and triathlete (2 70.3s), herself.