There is a chance you could be pregnant. The pulling out method does not always work. The precum (majority of the time) will get you. but as you also stated it could be very well due to stress and excitement for your graduation weekend. Congratulations on your graduation and keep up the good work :)
take a pregnancy test, if you are late for your period a test should show positive by now, if it's negative then you are most likely late due to stress.
It is possible you could be pregnant because the pull out method is not a good form of birth control. I know quite a few people that got pregnant that way. It could also be the stress but you should at least do a home pregnancy test. If you were due last week you should get a positive on that by now. If you still don't get it by next week then call your MD. He may want to do a blood pregnancy test on you to check your levels or check your hormone levels. Good luck and I hope you get your answer soon.
Thank you guys for the comments. I do sometimes feel like I am about to start my period, but then it doesn't come. I don't really "feel" like I'm pregnant, but I still have doubts. I won't be able to get a pregnancy test until next Tuesday, so I guess I'll know then, if my period doesn't start. Thank you all, again.