Pain is more when get out of bed at morning. If stand or sit pain intensity reduces, but increases when lay down. During first five days of monthly period( MC)

Your stomach is way above your lower abdomen where your uterus and fallopian tubes are... is the pain really lower abdomen? Many women experience pain during their periods and find relief in various positions as you do. There are so many explanations depending on which of countless nerves are affected, the tilt of your uterus, the ability of menstrual fluid to flow, any possible fibroids or other tissue growths, hormone fluctuations, fluid retention, how you react to foods and liquids such as coffee during your periods - the list goes on. One exercise that helps many women is the pelvic tilt - lie on the floor and prop up your pelvis with your hands for as long as you can, several times.

Just to add to the above post,you should see your doctor to rule out any physical problems.

Its always best to seek advice from your doctor with physical,mental or emotional problem.