What Causes Delay In Periods?

Hi, I am 16 years old and I am 5 days late with my period. Last month, me and my boyfriend had sex on the last day of my period(5th day of my cycle) we used a condom and he pulled out. He filled the condom with water after and it had no leaks. Now, being 5 days late i am so scared. To the point where I can't even try to eat. My friends mom (who is a nurse) helped me out, she counted out the days from sex to ovulation and she said that there is no way that I am pregnant from that alone, we took a test anyway and it came out negative. I'm jus so scared, stressed and can't be pregnant. Please help :/ i have had slight cramping and an uneasy stomach. but i think its because im not eating and scared to death. also, i think i have a knot in my back? if that has to do with anything, but that could be from softball too.