Welcome to the forum. Menstrual irregularities may result cause of many problems, the commonest being low general health condition, weight gain or loss, polycystic ovarian syndrome
imbalances, genetic factors, pelvic diseases, stress
, tensions, etc.
It is nice to know that your thyroid levels are normal. Try not to gain weight, maintain your BMI, body weight index. Take nutritious diet, rich in iron, calcium, protein and minerals. Improve your general health conditions.
You can take vitamin supplements and a good uterine tonic like "Ashokariste" (ayurvedic) 4 tsp with water 3 times a day. There is no harm in trying other allopathic uterine tonics.
You should also get clinically examined, get USG, blood tests done to rule out any cyst or ovarian problem, fibroids,etc. Take care.