What Causes Delay In Menstruation Other Than Pregnancy?

HI, I was hoping you could help me, i'm 19 and i'm 5 days late coming on the period. i haven't had intercourse but there may be a chance that sperm touched the outside of my vagina 8 days ago. i have been worrying like crazy for the last few days. i have also recently gone on a diet and over the last 7 weeks lost 12.7kg. over the last 4 days i have been having cramps in my lower stomach but no period