Should I Be Worried For Missed Period After Protected Intercourse?

Hi was just wondering if someone could help me. I had sex with my boyfriend on the 7 th and 27 th of march,got my periods later that day on the 27 th.we practiced safe sex both times. My cycle is usually 28 days long,i haven't had sex since the mentioned days and its been 37 days and no sign of my period. I took a pregnancy test after 7 days from the day i was supposed to get my period and the HPT was negative.I'm not on any form of birth control but i have been stressed about my finals and also have quit playing basketball with slight changes in my eating pattern. how worried should I be?what should i do next?