Hello,
The normal menstrual cycle lasts between 21 and 35 days. It can be due to irregular period which needs evaluation. The cause of irregular periods could be due to hormonal imbalance, stress, chronic illness, polycystic ovarian disease, thyroid gland disorders or diseases, pituitary gland tumors or cyst, galactorrhea, prolactinomas etc. You should consult a gynecologist and get examined and investigated if the irregularity in periods persists.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
I forgot to mention that I've been on Citalopram for my OCD for more than a year. But the menstrual irregularity dates even before I started the medication.