Please look into this:

Postural Orthostatic Tachycardia Syndrome (POTS)
Postural orthostatic tachycardia syndrome (POTS) is a condition that affects circulation (blood flow). POTS is a form of orthostatic intolerance, the development of symptoms that come on when standing up from a reclining position, and that may be relieved by sitting or lying back down. The primary symptom of an orthostatic intolerance is lightheadedness, fainting, and an uncomfortable, rapid increase in heartbeat.

https://my.clevelandclinic.org/health/diseases/...

And

Autonomic dysfunction in fibromyalgia syndrome: postural orthostatic tachycardia.
Staud R1.

Although fibromyalgia (FM) syndrome is defined by chronic widespread pain and tenderness, additional symptoms, including disabling fatigue and dizziness, are often reported by patients with this chronic illness. Although nonrestorative sleep may play an important role for chronic fatigue in FM, other mechanisms, including dysfunction of the autonomic nervous system (ANS), need to be considered. Many important biological functions, such as heart rate, blood pressure, respirations, and bowel function, are tightly regulated by the ANS. However, dysfunction of the ANS is common in FM and often becomes quite apparent after positional changes from supine to upright. Although such positional changes sometimes result in syncope, they are more often associated with palpitations and dizziness. Head-up tilt table testing can be used to evaluate autonomic dysfunction and is frequently helpful for the work-up of FM complaints, including fatigue, dizziness, and palpitations. One of the most common events experienced by FM patients during tilt table testing is postural orthostatic tachycardia syndrome, which is defined as a heart rate increase of more than 30 beats per minute after more than 3 minutes of standing upright.
https://www.ncbi.nlm.nih.gov/pubmed/19007537

I have had the same symptom that came with fibromyalgia. I used to be a long distance hiker and had an excellent heart. I wore the 7 day Holter monitor and my cardiologist diagnosed POTS (post orthostatic tachycardia syndrome). I now keep my salt levels high and wear compression socks. It’s worse when flared. Heat exacerbates it tremendously. My cardiologist explained that it happens when nerves can no longer constrict the blood vessels so that the blood simply follows gravity—so I would say it’s absolutely related. When the head doesn’t have enough blood the heart works hard to recirculate it. When it’s bad, lying down with legs and arms up helps, but cold showers help me recover the quickest.

i did. fibromyalgia affects the heart as it a MUSCLE. I found also it was too much Magnesium, try getting off that ? it can also be anxiety, which I also had. takinf anti- anxiety meds now - doesnt happen anymore.

I do. It only does this when I'm hurting. Even getting off the couch makes it do this. It makes me very lightheaded when this happens. I also have strange fast beats that are so strong you can see my heart pounding. It will stop and triple beat again. It's really bad when I'm pregnant. Still don't know why it does this.

Thank you both so much for your extremely helpful information. I’ll definitely mention this at my next visit. It’s just so frustrating to have new symptoms coming up and not getting answers.