what is a thick uterine lining? heavy periods?

I have painful heavy periods and my aunt said both her and my grandma have "thick uterine linings" I went to the doctor before that to get bc but it didn't change my flow and painful cramps. It actually made my PMS worse. Horomones. They found out I AM anemic.does anyone know what causes thick uterine lining, or what's going on here? I'm my period is literally ruining my life 😅