What Causes Miscarriage?

As a 42 year old female who has experienced an ectopic pregnancy 18 years ago, 2 successful pregnancies. I experienced some spotting, bleeding for 1 week. I took a pregnancy test and it was positive. Heavy bleeding at the weekend with clotting, visit to hospital, vaginal scan and bloods, hospital confirmed no pregnancy diagonsed early miscarriage. HCG level was 30, repeated today level was 44, why can the level rise if i have miscarried and i am still experiencing mild pain in my right side.