How did you react to her freaking out? Did you coo and coddle her, baby talk to her, and try to coax her out of hiding?
If she DOES react that way again, completely ignore the behavior and wait til she calms down to give her praise and attention. Coddling over her when she's acting afraid will reinforce the fearful response and make it stronger and thus harder to get rid of and deal with.
I'm not sure what may have caused it (and she may be going through a "fear period" but I'm not sure if the ages match up, someone else on the forum will likely be able to help you with that), but just make sure she continues to get more and more socialization and that each experience with a new person is positive (no yelling, no physical corrections, just lots of verbal praise, playtime, and treats!). You wouldn't want one bad event ruining her well mannered and confident personality. Get her out into the world. People coming into your yard and house aren't going to get her enough socialization. She needs to visit stores, busy streets, see traffic and playgrounds and parks and the inside of different buildings and pet stores, bring her to friend's houses if you can, and get her to meet as many dog-friendly dogs, well mannered children, and adults as possible. Get her to meet people of different races, who where different clothes and hats, people with beards, mustaches, who are overweight, carry umbreallas, are really tall, etc. A lot of socialization will make her much more confident as an adult.
I would have Beth come over more often if possible, and have your pup meet her a few more times and introduce them slowly. Like I said - ignore the fear response and wait to pay attention to her until she's being calm. And make sure you yourself aren't behaving nervously, dogs can pick up on your nervousness or anxiety and that will make them think there really IS something to be afraid of! It can really make a difference if you act calm, confident, at ease, and in control.