now thats a new one. honestly,this just does not sound at all like just "overworking" the arm. this sounds like a hand/muscle tendon thing. have you actually seen a doc about this just to get HIS evaluated guess as to what this could be? the mere fact hat this thing is causing your finger and thumb to 'lock up' in some way would usually indicate some problem with tendon or muscle in there. that lump could just be part of a tendon or muscle,or it could be some benign type of growth like a ganglion cyst or something?
but whatever this thing is and what it is causing for you as far as symptoms just really needs to be found out. seeing your doc to just have this better looked at and have him run your wrist and hand/fingers thru all of the normal ROMS would help narrow down possibilities. he could simply send you for an MRI of the hand itself too. that would really be the quickest and easiest way of getting that important inner look there as to what this actually consists of anyway. but i would at the very least here, make an appt with your primary for now just to get this evaled properly and sent for some tests if he also does not know just what this actually is either. but it would appear based upon your description that it IS involving at least to some degree one of or part of possible muscle/tendon/ligaments or you simply would NOT be having any level of "lock up' goin on there ya know? this could possibly be some level of actual compromise of the nerve that runs along the radial(thumb side) side of the wrist too that governs those muscles. like i said, that MRI really would be a great way to simply 'see" in there.
i would make that appt with your doc tho. this realistically would be the only true way of getting to what this actually is and what is causing it. i wish you luck here in getting that important dx sooner vs later. for the "sooner" i seriously would push for that MRI. please keep me posted as to what you find out. FB
