Can Dogs Spot While Their Pregnant?

My Dog Had puppies about a year back, And we kep a boy. We later got him fixed. winter cam so we started keepi them iside he their indoor kennel. She had gotten out a cople of times during the day while i was away at school. She later ad her "period". and we recently think she is Pregnant, because she is a skiny dog nd getting fat fast. Bu we dont know if its possible cause she had h "period". All im asking is, Can dogs Spot blood while pregnant??