The left side of my left ankle has been red & swollen for three days now. It is painful to the touch & hurts to put weight on it. What is wrong?

Ankle Fracture? First, get an x-ray to rule out a fracture (if there was trauma causing the swelling and redness) if a fracture had been ruled outby x-ray then a thorough exam of the ankle by a physical medicine & rehabilitation physician or orthopedist physician is recommended.
Infection vs injury. Pain, redness and swelling are the main signs that you may have an infection in the localized area. Especially if there is no history of an injury. Gout and certain inflamatory diseases can present this way. See a doctor to find out which it is.
Have it evaluated. Heel pain could be due to different reasons, of which may include: trauma (fracture, tendon or ligamentous tear, arthritis), infection, metabolic conditions (gout) musculoskeletal or biomechanical abnormality, or neurologic conditions (neuropathy). A thorough history and physical exam and possibly laboratory and radiology exams could determine the cause. If symptoms persist get it evaluated.
Ankle Pain. It cold be many things like gout, sprain, infection or fx even soft tissue damage. Seek out help from podiatrist.
Ankle pain. Could be many things, sprain, trauma, tendon injury, inflammation, fracture, stress fracture, bug bite, allergy, infection, charcot foot, etc. What else is the history behind the symptoms, what have you tried for treatment? Try rest, ice, compression, and elevation, unless you have vascular issues or fever, chills, nausea or signs of active bacterial infection (red streaking, pus, drainage).