One of my athletes, we'll call her "Kathleen"... because that's her name, has been experiencing pain in her Iliac Crest. The model below points to the area.. this is not Kathleen's actual pelvis. I'm also not a doctor so this 'diagnosis' is based solely on my familiarity with anatomy in regards to the running motion. This is a common sense starting point in solving the problem. Kathleen is not injured- she is just experiencing discomfort in the area described.
She experiences pain on both sides soon after cycling and running. The quadratus lumborum can be overloaded during running and either be used as a hip abductor and or a hip extensor. When you get fatigued and you have either weak gluteus max or weak gluteus medius (hip abductor) you can over use synergistic muscles such as the quadratus lumborum during activity. Since this muscle is not the primary mover the muscle gets overloaded and strained.
The above illustration allows us to see where the attachment of quadratus lumborum is located.. which is where she is feeling the discomfort. Kathleen was a bodybuilder for a lot of years so it's hard to imagine that she may have a weak muscle group. This also is an example of why core work is not only such a broad thought, but why it is so critical to running (and cycling).
Having Kathleen focus a little more on her abductors and psoas focused core work is where I'll start with her. Avulsions can occur at this site so the first thing I am having her do is rest just to prevent a full blown injury.
Here is one of my videos where I have shown a stretch cord exercise that targets the abductors and the gluteus maximus..