Hmmm....

I am definitely no expert, and I have never dealt with the same problem before, but at 15 months she is probably going through a lot of maturing. She is no longer an outgoing, trusting puppy. It seems like a very reasonable age for her to start becoming more wary of strangers approaching her, and it doesn't sound that unusual to me. Fozzie became much less trusting of strangers at around her age, as did Gonzo.

I would work on desensitizing her to various people approaching her from ALL directions. She should be allowed to look at them, but she definitely shouldn't be excessively skittish.