First things first - don't feel guilty. Its going to add another negative emotion in there and add to your burden.

I think that babies can understand the tone of your voice, but he is definitely not being naughty, hun. He may be testing his boundaries which is completely normal - thats how he finds out what they are. Everything you said is normal behaviour in my opinion, especially while teething. Maybe next time you can leave the room and just close the door on yourself for a mniute or two - even if its in the toilet? At least you won't have yelled and fely guilty. He definitely is not going to understand "Don't bite Mummy" but he may understand by the tone of your voice that its not acceptable.

Can someone watch him while you have a nap?