Sudden pain on back of thigh only when walking or using stairs not at rest. the pain is at the very bottom of left thigh, just before knee crease almost to the side of thigh but not quite. small area, about size of a nickle. sharp pain. ideas?

It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some tests. It could be a baker cyst though popliteal artery aneurysm would need to be excluded. It would be prudent to see a doctor, start with a primary care provider for an assessment and to ascertain if referral to a specialist may be needed. Wish you good health!