I don't understand....

I don't understand why it is so easy for some people to get pregnant, yet so hard for others. Or even why it is so easy for a person to get pregnant one time, but not another. I just want to be pregnant. I want another baby so badly, it hurts. I can feel it more and more as the triplets get older and older. They are not babies anymore, and I hate that! At my last Dr appointment, they said the ultrasound was clear, and everything looked fine. Well then why am I bleeding on cycle day 15???? I should be ovulating soon, and instead I am bleeding. I don't understand my body. Why can't it just work normally, the way it is supposed to. I try not to think about it, I try not to worry, I try to remind myself that it will happen when it is supposed to happen. But what if it isn't supposed to happen? What if it never happens? I can't even think about that. I hate not knowing, I hate not being in control.