What could be the reason for lower back pain that radiates to the sciatic nerve upon certain movements?

Hello doctor,

I am 23 years old and have had lower back issues for the past three years. Sometimes it gets severe. I have been active in weight lifting and running my whole life. Recently my lower back has been in constant serious pain. I sleep with a pillow between my legs, but it does not help. I wake up in pain, and stretching is also not helping. It radiates into my sciatic nerve with certain movements. Also, the main trigger for this pain that makes it excruciating is when I sit up straight in a chair and lift one leg straight up and bend my head down. Kindly help.