HELP!

Over the past week my 9-month-old son has started to show signs of aggression. Last Thursday night I took him out of the car and accidentally knocked his head slightly, he then screamed (not a crying scream) pulled my hair, head butted me and then tried to bite my face! I shrugged it off until this morning when he did the same again when I accidentally scratched him changing his nappy. (I don't usually have this many accidents).
I'm in tears writing this. I'm so worried there is something wrong. I don't understand how a child so young can show such aggression and frustration. Maybe I have done something wrong? Until these two occasions he has always been so placid. He is always so content, he never cries and is always happy.

Any ideas or comments would be great!
Thanx guys
xx