Definitely sounds like anxiety. I had a similar thing happen to me a week ago. I did some minor workouts before bed and as soon as I laid down, anxiety and reflux attacked me hard. It surely sounds like tension in your neck, it pulls everything tight and makes you feel heavy in the back of your head.
FYI... You shouldn't be straining your neck to do crunches, those are to work your core muscles. If your core muscles aren't up to crunches (common, no worries), try planks or squats to build them up.
Most of all, relax. If you feel anxiety symptoms coming on, don't push your body hard, it's already in duress and tension. Do things that trigger a relaxation response, deep breaths, lay down, soak in the tub, etc.