After sitting and rising left leg threatens to collapse. Once several steps. All ok some pain in calf?

Leg weakness. This scenario suggests extension related or change in posture related nerve compression. If persistent should be evaluated.
Possible knee derang. One must investigate the possiblity of an internal derangement in the knee. Pathology such as a meniscal tear(most common) or loose body might also cause such symtoms. Also evaluation for possible patella instability might be helpful.