Why does my dog keep growling at me?

I have a dog who is a mix between a chihuahua and something else. He is around a year old, give or take a month or two. We've had him for around 2-4 months. When we first got him he got along fine with everybody, but recently (within the past 2 weeks or so), he has been acting strangely. Some things he has been doing: he'll be ok, then he'll start yiping and running likes he's hurt. This is usually accompanied by him shaking. Other times he'll growl and start to shake if somebody tries to pet him. He knows none of us would intentionally hurt him, but still he acts as though he is afraid of us. Why is this, and how can I get him to quit growling at us? I understand smaller dogs are timid, so I'm not sure if the shaking can be helped, but I do not like him growling at us.