It does sound as a muscolo-skeletal problem possibly supported by stress-related issues (maybe coming from work maybe coming from the fact you are now getting a bit pissed off about this problem and I do understand you) and who knows, maybe your posture and your ergonomics (position of your PC and monitor, poor chair, typing and answering at the phone holding it always in a same fashion, ecc...) at work are not brilliant and that surely doesn't help.
You may be interested from the fact that cortisol, adrenaline (stress related hormones), inflammatory substances/waste product coming from a constantly hypercontracted sub-occipital muscles/Cervical erector spinae, may not be "washed away" by the blood, as hypercontracted muscles constrict vessels and that prevent the blood stream to optimally nourish the tissue which may have an important effect on hypersensitise both peripherally and centrally your nervous system and the local tissue too.
Carry on doing these exercise that, I guess a colleague, gave you since they seem to give you benefit. Carry on with heat packing if those help too. Find some time to relax and chill out.
You may consider a visit to a osteopath/chiropractor or a physio expert in spinal manipulation, since during that time you were working so heavily (12 hours is madness!!) you may have altered badly your spinal dynamics and possibly you may have some chronic somatic dysfunction at the level of your cervical facet joints. Irritation of these facets joint, by either physical, mechanical or chemical (hormones and so on) stimuli can easily ending up in sustaining and supporting tensions in the above muscle groups.
Hope that helped to clarify your doubts a bit
wish you all the best
Emanuele