What could be the cause of my sudden chest pain? I am just lying down. Something like a tightened rubberband pressure in center

Depends. Could still be heart. Could be heartburn (esophagus with reflux). Could be lungs and pulmonary embolus. Could be muscle spasms. If persists or worsens, see md or call 911. ..
Sudden chest pain. I am amazed you are asking me this question. I do not know your age but any chest pain serious because you have to make sur it is not cardiac. By which I mean you may be getting a heart attack, even if it is not heart attack but you need to be evaluated to rule it out. Other causes of chest pain can be esopahageal teflux or chest wall pain. So if you have already not done see your doctor or go to er.