Why would my period be so late?!

Constipation, headaches, breast tenderness are symptoms of pregnancy but ALSO signs of your menstrual cycle/PMS and stress can add to it and change it too. It depends on let's say your mom gets her period late because she's stressed, and it's happened to her a lot, it could be hereditary that you get yours late also because you're stressed. Personal experience; my mom and I, when we're stressed we usually get our periods early due to stress which also causes us to bleed a lot more. You can find ways to calculate online to be more sure if you are pregnant or not by finding out when you were ovulating and IF you conceived. It would also help knowing when you last had sex, if there were any mishaps etc.

Personal Experience. Education. Health. Internet Research.

You're right, it's probably just stress. Try to relax and drink plenty of water. Your period will come soon enough. Mine often skip months because of stress and being irregular. I'm sure your's will get on track soon. If you're still concerned, go take a visit to your obgyn :)
I agree with Sara- take another test in a week. Make sure you use a good pregnancy test and take it as soon as you wake up in the morning to get the most correct results.

It is very possible it could be from stress. I know this from personal experience. But wait a day or two and take another pregnancy test but be sure to use first morning urine because it has the most pregnancy hormones in it! Good luck girl! :)