Depends. Heaviness in the legs can have several causes, including circulation problems ("vascular claudication"), nerve-related issues ("neurogenic claudication"), muscle fatigue, muscle conditions, and general deconditioning. If your problem persists, it is worth visiting with your doctor. Take note of any symptoms affecting your arms or your bladder-- that can help the doctor sort through your problem.
Seek medical attn. "heaviness" in a limb suggests a problem with nerve or muscle function, potentially due to a serious underlying condition. Involvement of bladder or bowel function, and/or difficulty walking would raise the level of concern. Evaluation in an emergency room or urgent care facility would be best, as a delay in diagnosis could result in lifelong impairment or worse.
Cervical stenosis. Narrowimg of tje spinal camal slowly in neck region can do that usually with poor balance.
See A Dr. Very difficult to know. Depending on your previous history and medication list many things are possible. From congestive heart failure, to a neuropathy, miopathy, arthritis etc.