Don't think it's tendonitis.
See if you can feel around in your hips, upper legs, and lower legs for any tight muscles. Could be hammies, IT band, quads, other muscles in the hip, etc.
Find them and then massage/foam roll/tennis ball them away.
Ice if there's any swelling... heck, if it's patella ice after any exercise.
Report back in a couple days if there's any improvement or anything else.
If you were to see a doc you should seen an ortho.