I've had a knot on left side of my neck for about 2 years. I've always thought it to be a tense muscle because I have fibromyalgia. Suggestions?

Possibly a lymphnode. Just have you primary doc check to see if it is a muscle or a node. The pe should be all that is needed. If he or she has any doubts they will order a follow up study to clear up any issues. Fm is the curse of humanity, you need a wellness and self-care program. Start w self/pro massage, chiropractor, heat, epsom soaking, stretching. I use acupuncture!
Heat and posture. Muscle knots are harder than anticipated. You must find cause of knot and treat. Could be sleeping position, work, carrying purse, stress, or physiologic problem. Try heat, massage, stretching. Can also use a tennis ball and lie on floor with tennis ball pushing on knot to massage. Keep you'd shoulders back as much as possible through day.