Hi snanny - There are many causes of sacroiliac joint pain. As Ann noted in her article, the causes include trauma and pregnancy, as well as biomechanical causes such as muscle imbalance, overpronation, or leg length discrepancy. Additional causes include osteoarthritis, gout, rheumatoid arthritis, psoriasis and ankylosing spondylitis.
In terms of your doctor saying you are too young for arthritis, you're correct to question that statement. Arthritis can affect anyone, at any age. You can learn more from the Arthritis Foundation: http://www.arthritis.org/learn-about-arthritis.php
Is this information helpful? Please let us know and if you have additional questions. Thanks, Pat