I shouted at my boss. how awful is it?

Okay so, i work a part time job at an insurance company. I've worked on Saturday and I'm supposed to start at 10:00. The guy who is responsible for the part timers wants us to be there at 9:45, it's nothing official everyone else is really flexible with the time. Yesterday i was there at 9:53 because the traffic was so bad, and he wanted to give me a warning but i shouted at him a bit. He said it was okay. To be honest I want to attend up to myself more, because everyone treats me pretty badly, and i was honestly upset, because I'm otherwise never late, and i really can't influence the traffic. I don't think it could have any consequences since the official starting time of my schedule is 10:00 and i was there. I just feel really bad for not containing my frustration.