Hi Grad of 2013,
Is this a normal period for you? At the right time, heavy to moderate bleeding, red blood, etc. This would be around the time that you should be expecting your period. Also, usually when you have sex, like on the 1st of last month and proceed to get your period you are not pregnant. If you have sex and don't get your period chances are you are pregnant. That is a rule thumb a lot of ladies use regarding pregnancy.
If you are still concerned that you might be go ahead and test on a home pregnancy test with the first urine of the day. Chances are it will be a waste of dollars but it might help give you peace of mind.
Hope this helps and good luck. Congratulations on your upcoming grad. Have fun and be safe.
P.S. Think about adding some contraceptives to your daily routine so that you don't get any more pregnancy scares!