Is she fixed? Could she be coming into heat?
It is possible that one of the people was incredibly fearful of the dog, and in an effort to master their fear, some people like to confront it by meeting the dog. They usually do this somewhat tentatively, while staring into the dog's eyes. What the dog senses is actually unknown, but that they sense some terrible negative feeling and usually a challenge when this happens, is pretty clear.
On the other hand, the people could have been perfectly benign and your bitch may be having a seriously off day. Even a vaccine reaction, considering we usually give rabies around 6 months and then again about 18 months, your girl is 20 months, I wonder... I hope not.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC