If she's not been spayed and was in heat she shouldn't have been allowed outside off leash or unattended. Of course, your neighbours shouldn't have intact males roaming free as well.

Is this her first litter? How old is she? Is she a small breed compared to the males that you saw with her? You'll need to take her to a vet and have this information available as depending on the circumstances it could mean it's safer to take the puppies by c-section or if she's not far along to abort the litter for her safety.

Whatever happens, please consider having her spayed so you can avoid unwanted and unplanned puppies in the future.