It can be hard to tell. Like what was said earlier it could be an impending M/C or it could just be bleeding and be just fine. What you would be more worried about it heavy bleeding, especially with cramping and passing large clots.
that's exactly what happened to me with my first miscarriage. the bleeding would start and stop and start again till it eventually got heavier. it may be an impending miscarriage but then again everyone is different and it could be nothing. i hate bleeding during pregnancy coz you never can tell whats going on. i had some pink spotting around 12 weeks with this one and everything turned out fine im now 6 months along. I hope everything goes well for you and its nothing!!