Hello Rachel, I'm Dr. Deb. Thanks for requesting me; I'll do my best to help you today.
I'm sorry for this concern for your girl. There are two possible explanations for her behavior:
1. She could be pregnant since her behavior and milk production is consistent with it. Gestation is about 63 days so the timing is right if she got out of the yard and met up with a suitor.
In most cases, a pregnant dog will produce milk about 1 week prior to delivery...not 2-4 weeks so this doesn't fit.
And most pregnant dogs don't nest for one week either but usually for a much shorter period of time.
It is very difficult to auscultate heart beats of puppies especially if there's only one. An x-ray would certainly confirm pregnancy at this point.... but more than likely,
2. She's having a false pregnancy (aka pseudocyesis). This is a condition where females will experience all of the hormone changes associated with pregnancy and yet there are no puppies to be produced.
This LINK describes this process in some detail which may be of help in understanding this condition.
This condition isn't harmful in any way and I don't typically treat it. She may continue to produce milk if she's licking her mammary glands so she may need to wear a cone or t-shirt to prevent access to them.
If you plan to breed her, then I'd do so at her next heat cycle since she may go through another false pregnancy if you don't. Of you could spay her after this false heat is over if you don't plan to breed her.
I hope this helps to reassure you that her behavior is quite normal. Deb