Changes are normal in dogs as they mature. In any breed and regardless of whether they are spayed/neutered, they go through a maturation period at 6-12 months and another at 24-36 months. Breeds that are stand-offish, one-person, guard-suitable, and loners can be just as friendly and outgoing as love-everyone dogs are when puppies. If you got your dog from a breeder, what were the parents like? Chihuahuas are a breed with vast differences in temperament, and they do tend to inherit the behavioural patterns of their parents. If you got your dog from a rescue or shelter, you would not have had the opportunity to choose one temperament type over another.
However, there are other possibilities as well, and it is important to have a good idea of what is causing your chi's behaviour before I offer suggestions on how to work through it.
A second possibility is a health issue. You will know for humans, when you are ill or injured, it can make you quite grumpy, and the same is true for dogs. If the problem developed suddenly and immediately following or shortly after her spay surgery, there could be complications that are bothering her or even an irritation due to the stitches. How long ago was the spay done, when did the behavioural issues start, and have you seen the vet since the spay was done?
A third possibility is a traumatic event. Some dogs only react to actual physical harm, but chihuahuas are a highly intelligent and sensitive breed who can be traumatized simply by fear. Can you think of any event where she was injured or frightened, or are there times when she was looked after by someone else when you suspect something might have happened?
Can you also tell me her size (as chihuahuas do vary a LOT), how long she is left home alone in a typical day, and whether there are any children who live with you or visit you.