Puppy aggression??

[NielZ]

Sorry if this post is long but I feel like I need to include a lot of info...Yesterday night we had a very scary experience with our 5 month old Border Collie puppy. It was around 9:30 at night and he was laying on the floor by the refrigerator and I went to pet him on his back/hind legs and he snapped. He lunged at me and bit me on the side of my chest. He showed no warning signs. After this incident he did not let me or my fiance near him without snarling, baring his teeth and lunging at us or trying to bite us for about an hour. It was like he was terrified of us for absolutely no reason. We were able to calm him down by sitting on the floor with him and gently petting him and not letting him bite is and basically just holding him. We don't know if he was hurt, or scared of the thunder since we had a lot of thunderstorms yesterday or if he was exhausted since we had a long day. Earlier in the day we took him to the dog park, and then my fiance took him for a run a few hours later. He then he came to my parents house for a few hours and then a stopped by a friends house for a bit. The whole time he did not show any signs of aggressive behavior. He is familiar with my parents house and all of the people there so its not like he was uncomfortable or anything. My 2 year old niece was playing with him (supervised) and he was sooo good with her. It was like a we flipped a switch out of nowhere and we cant figure out what happened. The only thing we could think of was that he was scared of the thunder and freaked out or he hurt himself/ he is teething and in pain. Is this possible? This morning he was fine, playful and cuddly as anything.