Every women's body is different and they react differently to the hormonal changes. It is very much physiological.
However, there are conditions in which the periods become irregular. If the period is missed, first thing to rule out is pregnancy. If its delayed by 15-20days with then you should probable get an ultrasound to look for endometrial thickness or PCOS. Other than that you can get
THYROID PROFILE done as thyroid hormone changes can result in period changes.
Next Steps
As I said there are multiple cause but you can as of now sit back and relax and see for a few more days. As you have stated in your question that you are not
pregnant for sure, you can just observe for few more days and we will see after 15 days.
Health Tips
Confirm that you are not
pregnant.
Get back after 15 days even if you get your periods