hey there.i read your query and understand your concern Firstly,please understand delayed periods occur in case of many healthy females and in most cases,the causes leading up to it will be trivial,so try to relax sice your history suggests no diet change/medical history /drug intake history many causes can be ruled out most common causes causing irregularities are either one/sometimes a combination of factors like stress,poor dietary habits,low body weight with or without over-exercise,thyroid disorders,other hormonal conditions called polycystic ovarian syndrome,small benign pituitary tumour and so on. i would recommend you to relax and cut down stress(which is the culprit in case of most young women along with thyroid disorders), do the thyroid function test to check for the levels, check hormones levels like prolactin,FSH,LH etc ,do ultrasound abdomen to rule out PCOS, and maintain healthy weight and diet in most cases,emotional events and stress along with thyroid disorders cause this and all this ccan be easily taken care of. so relax,&hope i have clarified. follow up if needed.good day