Go to your GP for a referral to a specialist, as it may be a serious condition needing immediate treatment.
for my 2 cents worth: Sounds like a work related neck problem causing nerve irritation, which makes you very sore as the muscle tighten up-classic tension headache. The hot conditions bother you, causing a stress response, and you tighten your muscles in the neck and head.
Try some muscle relaxants-that should give you an indication.
Do you hold your neck at awkward angles for long periods? We all suffer wear and tear, and necks wear out. Some occupations are harder on necks than others.
Think also about your other parts of the day-how are you treating your neck then?
Poor seats, computers etc can also tip the scales.