Don't worry. Sometimes lifestyles issues and stress can cause this problem.
But it's better to get investigated in case there's a problem that can be easily treated, like hypothyroidism, PCOD etc.
Next Steps
Blood investigation:
Serum T3, T4,
TSH
Serum
FSH and
LH on the second day of menses.
Pelvic Ultrasound scan.
Health Tips
In case you feel your health is getting adversely affected, you should visit a gynaecologist after the above investigations