What Causes Delayed Periods With PMS?

Hi, I was wondering what could cause my period to be 8 days late. I was due for my period on June 30, but it still hasn't come. I have some acne, and my breasts have swollen slightly, and I have a constant backache. In the past month, I've exercised (but not rigorously), but I haven't been exercising much before then. Also, I just started a new job so maybe the stress is delaying my period?