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You have posted this question twice. I answered your other post. If you still have questions, feel free to ask.
To expand a little more, you would not be seeing clinical signs of heartworm disease at this age so you can pretty much rule this out.
If she is in heat, you will see swollen labia and she will leak small amounts of blood.
I would be most concerned about a parasite problem and suggest that you see the vet for a fecal exam as soon as you can.
Let me know if you still have questions.