One of the most common injuries, especially in Yoginis (a female Yoga practitioner), is at the Sacral Iliac joint. This is typically called the SI joint. A typical sign of the SI joint being out of alignment is a pain on one side of the sacrum about the size of a quarter. It is best to have a doctor check it out and give you a diagnosis.
If you have pain due to SI misalignment, try the follow exercises. I learned these last year from Doug Keller of Do Yoga. He is an amazing teacher filled with great wisdom and knowledge. If you ever have a chance to study with him, I highly recommend it :-) Please, remember this is my interpretation of what he said and has worked well when I've used it. However, these are not Doug's exact words.
CHECK WITH YOUR DOCTOR (I AM NOT A DOCTOR) PRIOR TO ANY EXERCISE PROGRAM!
Steps to healing from a misaligned SI involve: 1. Adjust the sacrum back into alignment 2. Tone the area to keep it aligned
Supine with bent knees and rock side to side. Cross your legs close together and rock. Swap sides. One side of the sacrum (if misaligned) will feel more "pokey".
Lengthen the leg on the "pokey" side out long and hold the opposite knee in. Bounce the straight leg up and down, keeping it off of the floor. Now slide the straight leg (still off of the floor) to the same side until the body begins to tip. Do this a few times. Only if neither side felt "pokey" and you feel no misalignment, then do both sides.
Another way to feel if one side is poking out, is to flatten the low back into the floor and feel the top of the sacrum.
To continue adjusting the sacrum, come into set bandhasana (bridge) with a block support just below the waistline. Using 2 wide blocks stacked on top of each other seems to work pretty well. Now bend the knees and place the feet on a wall or even straight up.
With all of this said, low back pain is frequently associated with stress, especially when dealing with money or security of any kind. Pay attention and notice when "spasms" occur.
In the past, I have dealt with back pain and had found that some was due to scoliosis, some a new hard bed, and some fear of not having enough money. It's only when I was able to address all 3 that healing began in earnest.