I have knee pain, can't bear weight, looks like bruising, but don't recall any acute injury?

Arthritis vs stress: Need too assess knee and x-ray your knee to get a better sense of your problem. See your orthopedic surgeon.

Take NSAIDS and wait: Try Aleve (naproxen) or motrin. Weight bear gradually over a few days. Make a mental note of where the pain is and what makes it worse. It could be many things, most of which are not surgical. If it lasts, see your doctor to determine if there is fluid on your knee. Special radiographic studies may be necessary. I usually inject a knee, because of the low cost compared to an mri.