Ditto what Ginger899 said. Any time there is unusual bleeding, a call to the vet should be the first course of action. The heat cycles could start to come further apart as a dog ages so it could just be her normal heat cycle, but if the bleeding is excessive, it bears looking into.
Ghilly
This could be urgent. Take her to the vet ASAP. If she is unspayed there is a possibility she could have pyometra which is an emergency.
In a normal heat blood loss amount can vary, but in my experience is rarely observed as 'heavy bleeding'