Why Does One Have Irregular Menstrual Cycles?

I have irregular menstrual cycles, my last period was 09/01/2011 and I know I ovulated on 07/03/2011, I took two pregnancy tests, negative on 20/01/2011 then a positive one on 24/03/2011, having called my doctor she told me that because they calculate the lmp I am 11 weeks pregnant, I know this is not the case but am confused how to work it out. I don't have a midwife appointment until 03/05/2011, I think I will be approx 10 weeks by then but as I don't know I am worried that this appointment is too late. Any advice would be most grateful. Many thanks.