I would take her to the vet ASAP to find out if she is in fact pregnant. If so, you can get an emergency spay/abort if you choose. Or she can have pups. But you need to know for sure if she is pregnant.
I would be concerned that you could be bringing unregistered, possibly mixed pups (could she have tied with any other pups while outside? There can be multiple fathers of one litter) into a world where there are so many pups, mixes and pure bred, that don't have homes.
If she were my dog and if she were pregnant, I would spay/abort for those reasons and your girls health. If you choose not to do that, some people here are experienced and can help.
If she isn't pregnant, PLEASE spay her. And for future reference, you shouldn't ever leave an unaltered female outside unsupervised because male dogs will do just about anything to get to a female in heat.
It sounds like SOMETHING is wrong, whether pregnancy or not.
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