Its basic ergonomics...the riding position we put ourselves in, unless perfectly upright, puts all the weight of our 10lb head on the small muscles of our lower neck and upper shoulders. even with the stock bars, you are in a slightly forward lean.
your head is too heavy for your muscles. you put a 10lb bowling ball on the end of a stick in your hand, tip it forward and see how long you can hold it
you are doing the same thing to those posterior neck muscles.
anti-inflamatory meds will do you good for just that, but they wont fix it.
if you have NO vertibrae misaligned, which only imaging will show, you will benefit from some form of shoulder and back exercises to strengthen the area between your shoulder blades to withstand the strain. Go see your physician....
(ex AT-C, PTA, LMP)