Dear Mark

From what you have written, I wouldnt say you have pinched a nerve, I think you may have a traction injury to your lower brachial plexus...

i say this because the area affected seems wide..the distribution is mainly in the C7/T1, the area where u feel parashthesia and pain down the upperlimb suggests this...also the triceps and pecs share that root supply (C7/T1) but they are supplied by different nerves, this is why a single pinched nerve is unlikely...the problem is coming from where they both share this supply...
My mind would be thinking the lower part of your neck...here you have the lower brachial plexus...a traction pull could tear it causing the type of pain and symptoms you describing...similar to erbs palsy in children...

other causes could be a prolapsed disc from between C7/T1(or a Wikipedia reference-linkfacet joint problem...unlikely because this would mean you have a degenerated spine, something not common with people your age)...it is unsusual to have multiple discs out except in the elderly or with serious trauma, also its usually the mid to upper lower discs that easily pop out (c4-c6)...however disc protrusion are common in your age group so this is another possibility...in addition, you say sitting affects it...sounds discy...

the fact the ibruprofen is helping indicates that the area is inflammed and that is getting better...

Can you describe the mechanism of your injury in detail...

In the interim you need a good assessment of your neck...this is where the problem is

cheers