What Causes Constant Pain In The Top Part Of The Back Of Neck?

I was wondering if you could help , I have a constant pain in the top part of the back of my neck and it can be sometimes feel heavy , my mouth is at the moment dry (though not always), I had a scan a year ago and nothing was seen, I have had physio but it didn't really help I am 52 and in good general health I am not overweight (weigh 86kgs & measure 6ft 2")