What Causes Delays In Menstruation?

Hi. I am 19 years old and had intercourse with my boyfriend around the first week of feb. We used protection. I was supposed to have my period around the 20th of february but have not had it yet. My breasts are tender but I was wondering if it could possibly be because my period could be coming. I don't know what to do. I do not know if I am pregnant. All I know is is that I do not have any other symptoms. Is there a most common symptom?