What Causes Delayed Periods?

I am 18 and my peroid is late, I am sexually active but safely, I noticed some blood in my stoll but just once so far. I always had my period on the 3rd week then it changed to the 2nd then 1st, I assume because of the girls I hang out with over time. However I just moved and dont have anyone around except my boyfriend, could I be late cause it changed back to the 3rd? I also get cramps for about 5 mins but they go away for hours.