http://www.dogclub.co.uk/advice/puppygrowth.php
That sounds like typical puppy fear period to me. Puppies often go through these stages where, all of a sudden, they react in a fearful manner to new situations, sounds and people and to ones they were previously fine with as well. I've provided a link and if you google you will find lots more. The key is to remain calm and try to avoid situations, like the one you describe above, until he is over it, as LavendarRott says. By avoid I mean not everything fearful, just ones like your description where he felt so overwhelmed he had to defend himself.
My Lab puppy went through a mini fear period at five months. I could tell because all of a sudden the refuse put out on the street on garbage day elicited wary backing off, slight woofs and avoidance. Previously the garbage had been a source of much interest, I assume due to the enticing smells. We just walked on by with me ignoring his behaviour in an attempt to not react and convince him it was nothing to be concerned about. You need to try to not reinforce negatively by coddling him but also not push him too much. Please google for links which will, I am sure, provide better explanations than I might.
My puppy is also getting "woofy" with strangers and doorbells and people we meet on walks sometimes. This I think is not fear but just him growing up and beginning to think he must be the master and define his position. He does tend to be pretty sure of himself. In these cases I implement some training. SIT, DOWN, WATCH ME. All clickered and treated for appropriate response. This for one, distracts him, two establishes me as the person in control and three gives him a reward for doing a correct thing.