Is foot numbness and lower leg weakness a nerve problem or a muscle problem?

Nerves: Typically numbness points to nerves. Numbness associated with weakness is concerning for spine disease (pinched nerves). You should consult a physician for evaluation sooner rather than later.

Depends: It could be either. Only an examination will determine what is causing the problem.

If this is indeed focal muscle weakness and numbness anatomically appropriate, this strongly implicates nerve pathology, either proximal or distal depending upon clinical anatomical findings. A muscular etiology such as piriformis syndrome could be one etiologic consideration.