Since when is this? Is there any history of injury or accident? Are the movements of the elbow painful and hampered / restricted? Are you diabetic? Do you have fever?
The injury ruptures the soft tissues (muscles and ligaments) and blood vessels. With the mode of injury, I would like to rule out the bone injury also. The blood vessel rupture leads to leakage and collection of the blood in the tissues, which can take significant time to built up. The redness and swelling is because of the collection of the blood called as tissue edema. Ice compresses would help. Do that for around 10 minutes every 4 hour. Take Advil or Motrin also. Try to keep it elevated. Going to ER or an urgent care would also be recommended.
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Best wishes and kindest regards.