Hi, My Name Is Lyn. I Am 53 Years Old.

Hi, my name is Lyn. I am 53 years old. My blood work was done 11/2014-good. My pap 5/2015-okay. I had a hormone test because my doctor kept telling me I was menopausal 2013- Nope, I was actually very fertile and healthy. Everything was good. In 3/2014 my doctor said I have a small fibroid -checked by ultrasound. In 4/2015 he performed another ultrasound and said the fibroid did not grow, but he wants me to have a biopsy because I have a dirty wall? I've had no problems with my gynecologist checkups since I was 18. I started having periods at 18 years old. I had 1 a year for several years and then sporatic. Then about early 40's I started having regular monthly periods and continue to. No pain. No blood inbetween periods. No cramping. No discharge. Regular type periods every month for the last 13 years. I'm healthy. I ride horses and exercise a lot. I eat right, mostly vegetables, fruits and chicken. I feel good. Skin good. No sores. Not bad nature. No uterine cancer in the family. Sister started period late and didn't go through menopause until mid-late 50's.
So, my two questions are: I am scheduled to have a uterus biopsy on or about the 2nd day of my period. Why? and IF it is because there may be cancer, can't a biopsy make it spread? I don't understand how a dirty wall (if that is even true) and a fibroid that hasn't grown (maybe even has been there for years and years) would constitute a biopsy.
Thank you for any help you can provide.