Why Does One Have Miscarriage?

It is about my girlfriend being pregnant. We are usually safe. She had her Depo shot (birth control shit) a few months ago which she had quit because she would not stop bleeding the first month she took it. She has not stopped since then. Its been about a month after and it is not bad at all it is little spots she says. So today we had sex and I had finished inside her and now she is worried without any protection she will become pregnant and I understand her it is reasonable. However she has problems down there in which she has had a miscarriage before she claims. I want to know the possibility of her becoming pregnant because I read when she is bleeding or on her cycle it becomes much less of a chance.