The eating patterns and stress could also be a reason behind your missed period, but as the above poster mentioned, you could still be pregnant if you had unprotected sex regardless of whether or not the guy ejaculated. Pre-ejaculate can contain small traces of sperm, and all it takes is one!
If the sex was unprotected even without ejaculation, then there's a risk of pregnancy. Wait till your a week late, you could take a hpt then. If your period still doesn't show up in a few weeks time, i recommend a blood pregnancy test. but then again, it can also be stress that led to delayed ovulation or no ovulation at all. hope this helps.