Gosh, it is hard to say. It could just be a strange period or it could be a really early miscarriage called a chemical miscarriage. Were you trying or anything? You 'could' go to your ob/gyn and they could do a hormone check on you. You'd still have HCG in your system potentially unless it is so early that it would be such a slight amount it wouldn't register. Did you ever get a positive pregnancy test or anything or is it the blood clot that has you thinking it could be a miscarriage?
According to WebMD, signs and symptoms of a miscarriage include spotting, vaginal bleeding that progresses from light to heavy, cramps, abdominal or back pain, fever, weakness, and/or vomiting. If you are concerned about miscarriage, you should see your OB-GYN, who can order lab tests (e.g., beta-hCG) and/or imaging tests (e.g., obstetrical ultrasound).