Pins and needles definitely sounds like nerve involvement... TrP referral pain doesn't cause that kind of reaction. Speed of the sensation is also important... nerve discomfort usually starts right away after the nerve is irritated while TrP pain usually spreads over time with pressure on the TrP.
Now you could be striking the radial nerve, but usually it's protected quite well by the triceps and teres major so I don't think it's that, but you'd have to rule it out.
What could be occurring, and what I find with some of my treatments, is while working the lateral boarder of the scapula you might be discovering a tight pectoralis on the anterior aspect of the shoulder which is causing a type of thoracic-outlet symptom to the arm. Usually to work the lateral scapula we need to protract the scapula posteriorly to access those muscles, which in turn puts pressure on the anterior aspect. If the pec is tight it will strap down on the brachial plexus causing the neurological short-circuit. Try treating the chest and anterior shoulder to see if it reduces or removes the discomfort.
Hope that helps, please post any updates here.
Steve