If you�ve eliminated the possibility of your menstrual cycle and pregnancy, my next thought would be the possibility of some scarring in the duodenum. Are you suffering from any reflux? Or do you find you�re often burping, or you have the feeling of heavy indigestion. Have your bowel movements changed recently. Are they different in consistency, such as tarry looking, black, or the appearance of being sticky? Share a bit more with us and we might be able to offer more specific thoughts. However, as we all state, it�s important to contact your GP. Besides numerous relatively innocuous solutions, there can also be more serious issues.
I wish you well
Fr Bill