There are a number of theories, but research has yet to pinpoint one particular thing.
Some GP’s believe a hormonal or chemical imbalance disrupts the way nerves signal pain. Others suggest a traumatic event or chronic stress triggers it. Or that it could be linked to a genetic predisposition, or infections or illnesses could trigger it off. Some say it could be caused by a lack of deep sleep or that lower levels of a brain neurotransmitter called ‘serotonin’ could lead to lowered pain thresholds or an increased sensitivity to pain.
Either way, it is definitely a combination of factors, rather than a single cause.