Everything crossed for you TVC.
Have you tried gently stretching the muscle ? I had a nasty fall off a horse a few years back and i was left with constant lower right back muscular pain. As part of my treatment i did rehabilitation pilates as my core muscles eventually became quite poor and one of the exercises i was given, which i still do today is a simple lie on your back, knees up, feet flat on the floor and move your knees together to the left, then the right. Need to do it for about 3 minutes.
This always loosens up that muscle nicely for me.