What Causes Abdominal Pain When Having Growth In Cervix?

My age is 36 and height 5 6 and I weigh 81 Kgs. I was having a growth in my cervix last year and after consulting the doctor recently and getting a sonography done, have come to know that the fibroid has reduced significantly in size. I am not on any medication but for the past 3 periods I have been experiencing acute pain in my abdomen which is unbearable and similar to a labour pain. Doctor has suggested to take pain killer for which I was not very willing but since the pain is too much to bear, I take pain killer. Wanted to know why do I have this pain which comes the second day of the period and lasts for some 10-15 hrs. I have two kids through c-section.