You are probably in need of an MRI, but your symptoms suggest compression of a nerve in the cervical area.
To self-test for this problem, lie flat on your back on the floor. Have someone apply axial traction to your head. This is gentle traction along the line of the spine, pulling the head up away from the body. If the symptoms immediately diminish you have positively identified the etiology. I doubt this is from the mouse use. Certainly posture at a computer has a role, however.
An MRI is useful to determine if you have a subluxion, displaced vertebra, or degenerative spinal disease. X-rays are inappropriate.
A treatment plan will depend on the differential diagnosis.
Generally an anti-inflammatory is prescribed and a program of intermitant axial traction for several minutes several times a day. If there is no degenerative disease range-of-motion exercises may be prescribed. A cervical collar is contraindicated because it causes loss of muscle tone.
My computer mouse hand is right hand. I did wait tables and held heavy serving trays in my left hand on a daily basis for over 5 years? I'm not sure if that could be just now coming to haunt me? I sleep with my hand in fists, that could be a factor...bad habit I've been trying to correct.
Is it in your computer mouse hand? If you're on the computer a lot, you might want to think about mouse placement and the design of the mouse. If you don't spend a lot of time on the computer, then consider your most time-consuming activity and how you are doing it. What position do you sleep in? I remember a friend who had recurrent hand pain with bruising, and it turned out she had a habit of throwing her arm over her head in her sleep. She was hitting her hand on the headboard.