I have general anxiety, and notice that after eating, and certain times a day my body strangely and suddenly go from tense to relaxed. head, jaw, shoulders tension just disappears. is this just anxiety, or could it be caused by other things?

Eating a meal causes release of hormones that can cause relaxation. In addition blood flow after a meal, as digestion takes place, is diverted from the brain to the intestinal tract and this causes relaxation & even drowsiness. In my opinion these mechanisms are related to what you experience.

It is not feasible to provide a meaningful opinion without taking additional history, physical examination and may be some tests. It would be prudent to see your doctor and discuss if you may be depressed. Counter-intuitive as it may seem, exercise may help. Start with low intensity exercise such as walking and gradually increase the intensity as tolerated. Drink more water. Wish you good health!