We'll we've found a place to start.
Try stretching your legs for a week and let me know if that helps.
Hamstrings attach to the ischial tuberosities (sit bones in your butt). If they're tight, they pull down on the bones which rotates your pelves backwards which flattens out the curve of your lower back. This causes your spinal erector muscles to work harder to maintain currect posture. They get overworked because a curved spine is much easier to stabilize (biomechanically).
If you'd like more details or how to go about stretching, or any other advice, PM me.