Why is my period a little late?

Okay, so my period was scheduled to come on the 5th. And its been 3 days. Okay, I've had sex like 2 months ago and he used a condom. But I also did get my period a few weeks later. Right on time. This time around I never got it. I have been working and my job is a little stressful. Worrying about my hours and everything that goes with working. I am also loosing weight. Since i've started i have been loosing weight. I am a little heavy. And also i was depo-provera shot for a couple of years. But it should have already been out of my system by now. Because I usually get a period once a month. And also I am on ADD medicine. And also i took a pregnancy test. Which it read negative..