I'd have her spayed immediately, for several reasons:
1. She's WAY too young/immature to be having babies
2. Who knows what other male dogs got to her?
3. Obviously, you have not done any research about breeding at ALL, otherwise
you would not be asking such a question!!!!
4. Anyone that would leave a defenseless little female PUPPY, in heat, tied up outside in the hot hot summer heat, has no business breeding anything.
5. You say that you don't want to risk her life, and that she's your BABY? Tying her up outside IS IS IS IS risking her life, no matter if she's in heat, or not. It only takes a fraction of a SECOND for a larger predator to get to her and kill her.
Please take her to the vet and have her spayed
