My thumb and some parts of my body (including legs and chest) twitch from time to time. I have health anxiety, can that cause BFS?

Yup. This is considered a common cause. Read: http://www.bfsrecovery.com/whatisbfs.php If this is the worst symptom - you might just learn to tolerate it - it's safe (if interesting). But if you have more troubling ones, consider good targeted psychotherapy. Best!