Totterdown. I agree totally with most of the previous replies. Get him to the vet asap. I had a similar incident with one of my cats, and it turned out that he had an abscess on his body just behind one of his front legs, which wasn't obvious by looking at him. He must have been in a lot of pain, and as I reached out to stroke him he sprang at me in the way your cat did. Trip to the vet, abscess cleaned up and antibiotics, he was soon back to his usual self. If your cat is in a lot of pain, the vet who is used to this kind of problem can sedate him for the examination. Hope all goes well. Just be careful until you have an answer from the vet.