Went To ER With Severe URQ Pain. Ultra Sound Was

Went to ER with severe URQ pain. Ultra sound was normal, CT with contrast showed small about of fluid consistency of blood in lining around liver. All enzyme levels normal. It was day 2 of my period. GI suggest liver hemangioma. Had MRI w contrast and say I have liver hematoma. Today (again 2nd day of period) have same severe pain. Have follow up with dr on Wednesday, but what could be causing this pain, and why always when I have my period?