Any part of your body is subject to irritation, swelling and pain if treated roughly or stimulated for too long. If your clitoris is stimulated too roughly and without adequate lubrication, it can become irritated and sore. Be sure you're well-lubricated from your own vaginal secretions or from a good lubricant, and try lighter stimulation. If you're still sore after trying this, you should consider seeing a gynecologist to determine whether you have an infection. Dr. J
You didn't mention whether your clitoris was directly stimulated or not. The clitoris is some highly sensitive tissue. If it was manually stimulated, it might have been a bit much for you this time. Try using some lube in the future. It will help cut down on the friction that can cause the swelling and pain that you are now experiencing.
Many women expirience this when they are sexual arroused. It is normal to see a reddening of the clitoris and a swollen look. This is due to the amount of blood that is being brought to this area of the body. Nothing to worry about. (I'm not a doctor though this is just my personal opinion and what I have heard from Talk Sex with Sue)