Possibilities for late period?

My gf has been under a lot of stress lately, with midterms and she is really nervous about getting into the college she wants. She usually gets them at the very end of each month, but last month it came a week early. Is that stress? And should we be nervous now because technically shes a week late based on last month? Although it usually doesn't til a day or two from now? We did have very brief unprotected one night but she was not ovulating and it was not much of anything because i didn't have a condom so we didn't let it last long.