You got negative pregnancy
test with normal USG scan. So you are not pregnant and it is confirmed.
Pregnancy is possible only, if unprotected sexual intercourse is done with ejaculation around ovulation time in regular period. Other days are safe and varies with high failure rate.
There are several reasons of delayed period like hormonal
imbalance, stress/ anxiety, ovarian cyst/ PCOD, thyroid
problem, wt gain/ extreme loss, irregular eating habits etc.
Need to investigate properly to rule out underlying pathology by Thyroid profile, LH/FSH, blood estrogen
etc. So consult with your doctor and take suggestive measures.
Avoid stress, take healthy diet, control body wt by regular exercise and proper sleep.
Practice safe sex by using safe period/ OCP/ Condom etc to avoid unwanted pregnancy.