What Causes Pain In The Fat Part Under My Thumb?

my hand has an ache in the fat part under my thumb. sometimes my arm aches as well but mostly my hand. at times my wrist right in the middle too. my thumb doesn't hurt to move, its fine. i do have just a little rotator cuff discomfort but only when i try to scratch my upper back with my left hand. would my rotator cuff somehow be connected to my had pain?