13 weeks after ankle sprain, was put under scotchcast for 3 weeks in beginning, now still pains on standing for more than 5 mins, pain in lower leg, swelling?

Sprain. You may have torn a couple of ligaments. Need MRI to determine and see a foot/ankle specialist.
May need more eval. Some ankle sprains don't heal with conservative care. In those instances, if indicated, advanced imaging and/or surgery may be recommended.
Re-evaluate . The ankle should be assessed again. A simple sprain should be essentially healed by now. You may need an MRI to check for bone and/or cartilage damage that was not seen initially. You may also have developed scar tissue that might benefit from physical therapy.