Having terrible pain starting in my feet and moving to my legs and knees, when on my feet. Not relieved by OTC meds, only helps to sit down?

Tendinitis. Make sure you are stretching the muscles in your legs from top to bottom (hip to heel) prior to standing in the morning and after rest. Do not walk barefooted or in flat shoes. Use arch supports. Watch your weight.... Ice massage your feet and use antiinflammatories with care and caution. You should see your podiatrist for further eval and reccs...
Biomechanical? This sounds like you may have a biomechanical issue. This would explain why you have the relief while resting. Look to see if you are flatfooted or if you "pronate". If you do, the answer would be to try and wear a good orthotic device or arch support to support the foot. When a person is flatfooted it casues a biomechanical imbalance where pressure is placed on legs and inner knees.
Plantar fasciitis. You could be experiencing plantar fasciitis which will cause you stand abnormally and put stress on other parts of your lower extremity. If otc not working then it is time to see a doctor for exam and treatment.