Best Answer

Need more information: Male or female?? How long after playing is this occuring?
Some small amounts of blood can be normal in a male dog who has been very active as bruising/stretching of the tiny blood vessels in the sheath can and do often bleed which shows up in the urine. Sand which entered the sheath can also cause small amounts of bleeding.
If this is the case and the dog is otherwise acting fine I would observe and contact a vet IF it continues more than overnight.

Toto, CD, RN, CGC answered on Oct 2nd.