Stress can be a big factor to delay a period. I've been here myself. The more you panic and stress about the what ifs of being pregnant then more likely it is to be late or even miss a period. I wouldn't worry untill you are at least a week or 2 late. You may of even just ovulated abit later this month. Try not to worry.
Depends. Have you had unprotected sex since your last period? If not, then it's likely late due to stress, illness or something like that