
A lot depends on what you mean by very heavily. If it is asy the amount normaly in a season diluted with the mating fluids then I wouldn't worry.
If it is real red blood in large quantities compared to heavy seasonal loss then I would want the vet to check she hasn't sustained an internal injury, has a polyp etc.
If she has perhaps sustained a tear then this couold become infected and may cause her or her potential litter problems.