Arthritis can be caused by many reasons. Some of the general types are:- 1. Osteo arthritis which is seen in old age 2. Rheumatoid Arthritis which is an auto immune disease 3. Gouty Arthritis 4. Septic Arthritis caused by infection etc.. Normally when people are over weight knee joints are the first to suffer. Common Sense treatments like weight reduction, Exercises, Balms, Gentle massages and hot water bags etc may help. If it is not releaved in a few days you may have to see a doctor to find out the reason.