I have been through this scenario countless times this year and am sort of going through it now. I get really bad headaches that just linger and linger as well as a stiff neck and shoulders. Whenever this happens I go to the physio and they tell me that my muscles are really tight. When i think about the headaches then guess what they get worse or don't go away and the same goes with nausea if i start to think hmmm i feel a bit sick then all of a sudden i feel like im going to vomit but never do!
Unfortunately with us it is all in our heads which is obvious but we don't believe it. Even I can say that yes its all in your head or vice versa but as soon as i get a headache i think brain tumour!
I have found that doing pilates once a week really helps and try my best not to think about it. My headaches always seem to start when im either bored and have nothing to occupy my time with, when i have had a bad nights sleep or when i suddenly think of a new symptom and think that i have cancer or something.
I know exactly what your going through. Try some simple stretches by putting your ear to your shoulder on both sides.