Thank you for the information,
What you likely have is called Olecranon Bursitis. This is swelling of the fluid filled sac that is located at the tip of the elbow. This sac helps to allow the skin to move fluidly over the underlying bone.
The sac can become inflamed and irritated for several reasons--infection (usually in this case, you will have redness, tenderness and pain), trauma/injury (such as a hard blow to the elbow), arthritis (rheumatoid or gout) or from prolonged pressure to the elbow (which may be the the source in your case).
Since it has only been a few days, this may resolve on its own with anti-inflammatories such as Ibuprofen or Aleve (if you can take these), ice application and elevation of the elbow.
If it does not improve over the next few days, see your doctor. The next step would be to remove the fluid with a needle. Your doctor will probably also inject a steroid at the same time. A pressure bandage is applied afterwards in order to help prevent reaccumulation of the fluid.
This is not usually a sign of something worse to come, so try not to worry.
Feel free to ask any follow up questions.