What Causes Anaemia?

Hello, I m 45 year old woman who has always had problems with anemia because of my menstral cycle. Three years ago I went to the doctors because I was tired all the time and was having chest pains from the simplest movements like walking leisurely . The doctor said my Iron levels were low and prescribed liquid Iron because the pills were suddenly making me sick and I wasn t taking them because of it., The liquid was worse. So I studied about different foods high in Iron. It s helped a little but I still feel lousy, tired and lazy. I have always been a very busy person always having to do something now I do hardly anything. I decided to take the Iron pills again because I could tell by the lightness of my stool that I wasn t getting enough. I took it before bed and it worked great I fell asleep before I could get sick, but all day today I have had very dark stools. I usually on go about once or twice a week. I m wondering if my body is not absorbing the Iron. Is that possible and if so what could be causing it?