Talk to Gynecologist on Delay Periods

Hello!! I'm 26 unmarried, and 65 kg, It's been 45 day's that i haven't got my periods, and 2 month's back i had cholecystectomy surgery. My cycle was normal but 6 months back i was facing UTI and recurrent periods but after that I had surgery and then after surgery my cycle was normal but now its been 45 day's I'm not getting my periods, Plus I'm taking anti depressents also (Zoloft 50mg) It's worrying me that I might gain weight, what could be the possible reason?