I Have Pain In My Upper Left Arm. No Initial

I have pain in my upper left arm. No initial trama accrued. It comes and goes somtimes every few minutes. It'll feel fine then it's a pulsing pain. It feels better resting and hanging at my side, rather than lifting my arm which increases the pain. I usally take a advil or aleve for pain killers. When I took the aleve I didn't notice much difference so the next day I took a aspirin and majority of the pain was gone. Any ideas to what is causing this?