Psoas muscle attaches to your lower spine and to your thigh bone. If it's tight (sitting too much?) it pulls your lower back. It's one of the common causes of lower BACK PAIN! If you can stretch this regularly then your lower back may feel a lot better!
It should be noted that these are TINY "knots" that feel more like rice crispies when frictioned over. The "large knots" that people usually point out are usually the part of a tight muscle that overlaps a bone.