First of all, you need to undergo one blood test
for beta-hCG and TVS scan to rule out possible conception.
Other reasons are hormonal
imbalance, vaginal/ pelvic infection
, old endometrial tissue, mental stress
, obesity etc.
Following initial test result, you need to consult with gynecologist to proceed other investigation and take suggestive measures accordingly.
Till then, maintain genital hygiene, avoid stress, take some anti-spasmodic medication etc. Hope it helps for you.