My dog is very sweet, but when we take her somewhere she gets scared and barks a lot and even..........?

Most chihuahua are very friendly as I should know I have a chihuahua mix. It's sounds like to me that just maybe something happened at the vet one day when you left her which is the reason for her behavior whether it involve a dog or someone who works there I don't know. Remember chihuahua's are very smart and don't forget anything