If the doctors have already ruled out physical probelms, then it is most likely to be related to your anxiety
than due to any physical problems. Non-specific symptoms like medically unexplainable pains are very common in anxiety. It can be distressing to the patient and the the usual physician's conclusion that "its all in the mind" does not help.
It is very important for you to understand that mind and body are intimately related. This mind-body relationship
is evident in day to day experiences, like feeling one's own heart best (palpitations) when afraid (a psychological phenomenon) or suffering from loose stools when anxious due to exams or some other reason.
Once your understanding of your problem improves, then you can discuss with your psychiatrist
in detail about things that might be the cause of your worry or anxiety. Adopt a healthy lifestyle with minimal stress and regular physical activity.