What Causes An Abdominal Pain Before Menstruation?

I have sharp lower abdominal pain in the middle I mean not left or right! And I should mention that I did ultrasound which showed everything to be normal! I don t know if it s necessary to be mentioned but I usually have these when I m near to my period... Please I need to know what it is and if it s serious or not! Ps I m a medical student I might have hypochondria!!! :D but this pain exists and it hurts