First of all it's OK to post about non-neurological things here. We even have non medical forums, so don't feel bad about posting. We're here to help if we can.
I see you posted this yesterday, so how are you feeling today? Did you end up going to the doctor?
It would be so difficult to isolate a cause for your swollen joint without looking at it, especially with the short history that you've given, but here is a short list of possible causes.
*Tenosynovitis and Bursitis
*Infection in a Joint - Septic Arthritis
*Gout and Pseudo gout
*Connective Tisue Diseases and Other Reactive Inflammations
The first 3 are the most common causes for the symptoms you've described but I wouldn't like to guess without seeing it and getting more background history. Sorry.
I hope you feel better soon.