A normal heat cycle in dogs typically lasts an average of 7 to 14 days. If a female dog's bleeding goes on longer than that, then it is worthy of wondering whether something's going on.
At times, some dogs may develop ovarian cysts, and these may cause irregularities in a dog's heat cycle.
Even more worrisome, bleeding after a heat can be indicative of Pyometra, a uterine infection which may turn life threatening if treatment is not sought in time. Affected dogs though are normally quite ill.
Other possibilities include the presence of a growth or tumor in the uterus that is bleeding. If the dog has been bred or accidentally mated, bleeding after a cycle may also be indicative of a miscarriage.
If a dog is bleeding/spotting after a heat cycle, it would, therefore, be important having her examined by a vet. The vet may wish to rule out serious disorders and perhaps run some blood tests to evaluate the dog's hormone levels.