Stress can absolutely cause your period to be late. The dark, brown spotting was most likely old blood. Blood will darken and brown when it is old and has been sitting in the body.
You can try another pregnancy test. HCG (human chorianic gondotropin) is the hormone that pregnancy tests check for. If the level is low the test will come back negative. It should show by this time. If the test is negative, relax and don't dwell on this. When you relax about it your period will probably start. If not, go to your gyn. Hope this helps.