I forgot to mention a really good exercise to relax tight lower back muscles is:
1) "Static back position" (Do a Google search)
You may have to lie there for 30 minutes before the back muscles let go.
Once your muscles relax you can try some hip strengthening exercises - keep your back RELAXED when doing these:
2) Put a pillow between your knees and squeeze (Knees are hip-width apart).
or
3) Wrap a yoga strap or belt around your legs just above your knees and press out. (knees are hip width apart).