There are multiple causes of joint and limb pain. One of the more concerning potential causes include infection.  If a joint is red and swollen in the absence of a specific trauma (and sometimes even if there was a slight trauma), an infection needs to be ruled out.  Other signs and symptoms of an infected joint include fevers and chills.  If you have or develop a red, swollen joint in the absence of a clear identifiable trauma you should visit your doctor that day.  If your doctor cannot see you, go to the emergency room.  If you have a fevers or chills and joint pain, go to the emergency room.  Failing to quickly identify and treat an infected joint could lead to loss of that joint and/or systemic infection.  

Of course, there are multiple other causes of a red, swollen ankle joint.  Gout, osteoarthritis, trauma and cellulitis are just a few.  A diagnosis cannot be given over the internet and it would be irresponsible of me to try for anyone.  To rule out serious causes, you should always have your doctor examine you in person.