Thank you for sharing your question with the EmpowHER community.
If you had protected sex, then you are not pregnant. Being 18 days late is quite a bit of time. Are you taking any medications, under an enormous amount of stress that could be causing your menstrual cycle to behave irregularly?
Most irregular periods and missed periods are typically do not signal anything serious and are often caused by a hormonal imbalance.
If this continues for 3 consecutive months it could be something called secondary amenorrhea and potentially Oligomenorrhea which is infrequent menstrual.
No Cycle or Missed Cycle
I hope this helps.