Endometrial Thickness, D&C, and Mirena.

Hello Doctor. I am 33 years old and have a 3-year-old baby boy. I was suffering from severe pelvic pain on the first day of my period for five months. Also, little nausea, constipation and bloating. I have normal bleeding during my periods. It's not heavy. The pain generally subsides from the second day. Still, I consulted gynecologists to get the solution for my period pain. They did a transvaginal scan and the result showed 9mm endometrial thickness on the 4th day of my period. So the doctor suggested that I undergo D&C and Mirena insertion. So on the same day I.e., on the 4th day of my period, I underwent D&C and also Mirena insertion. They have also sent a sample for Biopsy. I am a little worried about the Biopsy. Is there any chance of a serious problem due to endometrial thickness? What is the reason for severe pelvic pain on the first day of the period? Kindly suggest me.