Hi Claire, I'm going through a similar thing myself at the moment. It certainly feels terrifying, however, I've found some things that have really helped me:
1) Talk to your doctor. Don't feel silly about doing this, but I found that going to the doctor made me feel much better as he put my mind at ease.
2) Read all you can here about health anxieties similar to your own. It has really helped me to be able to learn more about it, the best thing is knowing that I'm not alone in these feelings.
3) Distract yourself. I find the same problem. I get these feelings at night. It's mainly because your mind isn't occupied and you have time to panic about every pain you get. So I distract myself with playing some video games (yes I am a big kid) or watching a movie.If you have an ipod or mp3 player listening to music while falling asleep or keeping the tv on for something to listen to really helps take my mind off it.
I get pains in my chest, in my arm, on my shoulders, between my shoulder blades, in my jaw and neck, but really it's just tension.When you're tense you are tensing your muscles (especially in the areas you are worried about) without realising.Deep breathing really helps and if all else fails just jump back on here and we'll all keep you company while you go through it