Possible miscarriage and the ER was not much help. Left still confused!!!

According to my LMP I am 6weeks6days. Last night I started having dark pink spotting and heavy cramps on both sides. Decided to wait until this morning to see if it got any better. I woke up to heavier spotting and really bad cramps. Went to the ER and after ultrasounds, pelvic exams, and blood tests everything with the baby looks fine and is measuring in at 6 weeks 4 days. But they still can't tell me why I'm spotting and could still be having a miscarriage. For some reason I just don't understand, if everything Looks fine, why am I spotting?