Those can be symptoms of both. You can either wait a few more weeks to see if your period returns when you're less stressed or get a pregnancy test to ease your worry. I would suggest getting the test done because you're only going to be more stressed, and that can delay your period even further.
If it turns out that you aren't pregnant, try to eat healthier foods during coursework and exam periods to counter the effects of stress. Check your vitamin levels as well. I always missed mine when stressed and found out I was deficient in a lot of vitamins.