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How long has this been going on. It is difficult to say --what I'm picturing may not be what you are trying to describe. The scenario you describe makes me wonder of some possible internal bleeding from the area of the armpit. This is a very vascular area with major arteries and veins. I would like to know the dimensions of the bruise you notice, any change in color and any known injury to your arm, under the arm, fall on the arm etc.
A high nondisplaced fracture of the humerus from say a fall onto an out-stretched arm might fit the symptoms as I picture them from your description.
Let me know further details and I'll be glad to get back with you.
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Anthony Bray MD