@feruze-- No, pregnancy isn't the only way you can develop SI joint dysfunction. There is actually several different factors that can affect your sacroiliac joints.
Did you fall or have any kind of accident before the pain started? A sacroiliac joint injury can easily cause dysfunction. And has your doctor checked for degenerative diseases like arthritis or rheumatism? Or were you checked for an infection?
All of these can lead to problems with the sacroiliac joint. Blood and urine tests, and an MRI can identify the cause. So I highly suggest you get these done if you haven't yet.