Lost the baby miscarriage at 6 weeks

I went to my doctor and they did an ultrasound and they told me that I'm losing the baby and gave me the option to go home and do it natural which is fine but it's been 3 days with heavy bleeding and gray clots. I was only 6 weeks pregnant but I'm so sad. This is my 3rd miscarriage I lost my other baby at 21 weeks also and one at 13 weeks. I just want to know why this keeps happening to me. What is wrong with me??? Any feedback will be great.....