stress. When ever i was late on my period i would stress it and think about it and that only made it come later and later. Just dont worry about it. If your unsure and want an answer get a p test.
It depends how long your cycles usually last. You ovulate mid cycle, so if you have a short cycle the it's possible to ovulate around cd 7 or 8 or so. Also sperm can live in there for 5-7 days so they could have already been in there when you ovulated and conceived. All this said of course, if you are actually pregnant. Have you taken a test already? Since you're a week late you can take one now, and get an accurate enough answer. Be sure to use first morning urine, since it's most concentrated and use a pink dye test. Good Luck, let us know how you get on.
It's possible especially if your periods are not far apart (i.e 28 days apart or less). Stress can cause the period to be late though, as well as dietary changes and if you've started a new workout program.
Your best bet is to get an over-the-counter test so that you can figure it out once and for all. If it's negative and you don't get your period for another week or so, contact your OB/GYN. They can induce a period if need be.