What are the causes of apendicitus? Could it be a junk food I had before I got severe pain?

No. The appendix is a blind ending tube shaped organ. In most adults appendicitis occurs when a hard piece of poop gets stuck in the opening of the appendix causing a blockage. There is really no way to prevent the occurrence of appendicitis.
No. Appendicitis is caused by a blockage of the appendix, an outpouching of the beginning of the colon. This can be exacerbated by some foods but this takes time and is not caused by one specific food, meal, or snack. When the appendix gets blocked, pressure builds up inside leading to inflammation. This can lead to infection and, in serious cases, bursting.
Several and no. Appendicitis can be caused by a few things including infection (vast majority) and cancer. Junk food is not a known cause for appendicitis.