I can tell you a couple of things it can be and I know from work and personal experience.
Muscle strain. There is big muscles that run there and you are tense or strained one, they will hurt. They actually get spasms in them and it will feel much better to lay on a heating pad. I get muscle tension alot there and it does hurt.
Other thing, gas pain. And it will drive you nuts as well. Both perfectly harmless, just annoying.
