You didn't say how old you are. But when I was in my 20s, I would even occasionally skip a period. It was a rare occurrence and always worried me (as it's doing you) but time would go by and another normal period would come, and things would be OK.
Stress has a direct effect on our hormones. Yes, just the fact that you've been eating less, exercising less and have had higher stress levels can result in changes in your cycle. Especially since you have had slightly irregular cycles since August, this doesn't seem especially worrisome.
If you were pregnant, you would be having some other symptoms by now. Things like tender or swollen breasts, nausea, fatigue and more frequent urination would have begun. Have you noticed anything besides the late period?
If I was in the exact situation you are, I would probably wait another couple of weeks until the 20th or so of January and see what happens (because if you skipped one completely, that's about when you'd expect to have one again), and if one doesn't come, check with your doctor.
Would you come back and update us?