How do I know my child has anger issues?

My son is 3 yrs old and he was sent home with a red rash burn on his forehead and a note from the teacher; he threw a car accross the roo and told twice to stop the teacher took it away and he repeatedly bashed his head on the floor. Now my son has not done this since he was just turning 2. He is usually really well behaved and very well manored. WHAT DO I DO? Help!