Argh, my neck!

Chiropractic (as in pure mechanical medicine of the body as opposed to the "alternative medicine" version) advice is probably not something best sought on the internet (I would even be hesitant about advice from a genuine physiotherapist/chiropractor; it really is something they need to see).

The two main areas are muscles/similar being too tense and bones not being correctly aligned (although muscles can cause bones and vice versa).
Bones really need to see someone and for muscles it would not be a bad idea, the main problem with muscles for me and people I know is they are not inclined to relax while you are in pain. Fortunately for me I know a few medics with some nice painkillers which allow such things to happen, most garden variety painkillers will not touch such things and I would not suggest messing with dosage/delivery without a medic present which courtesy of the line above.....

Don't worry, because it goes away within a day. At least it did for me. Hasn't happened since. Keep doing the heating pad, and trying to move it in small increments, and it will slowly get better within time.

Same thing happened to me before, it went away the next day.