Not sure if this should be in Litter Tray or Pets (honestly don't know the difference)
My brother's cat (a 3 year old male tabby) is the most placid and living of any car I've known. He comes to me all excited when he sees me, even paws madly at the glass door to get to me when I visit but then every so often, out of the blue he will just bite me.
Yesterday he was snuggled on my lap and I was stroking him (little purring motor was going) and then he just bit. I had a hoodie on so didn't feel a thing but it wasn't a nip he opened his mouth wide and tried to sink into my arm.
I Googled it and all searches point to "petting aggression" but there are times I'm not even touching him. Once he was just lying on his back near me with his tummy up and as soon as I stopped petting him he moved quickly and went for my arm. 
I love him to bits but this makes me on edge, it's not everytime I visit but enough for me to be slightly concerned, I don't want to be causing him stress if that is what I am doing? Help!