Seratonin/ hormone levels not sure help!

Please no judgments just after advice or similar experiences. Long story short- prior to getting pregnant I was emotionally up and down and my insecurities/ jealousy was ridiculous. When I was pregnant the entire time I was calm and felt very good it was a great feeling. I've noticed now in the last few months (daughter turned 1.5yrs old still BF almost out of milk though) my old self is returning and I'd say worse. Doctor has got me on sertraline AD and mental health plan to follow (ok to take while BF) I can't help but think my hormones and serotonin levels are all out! Not getting much sleep isn't helping. Thanks ladies your thoughts are appreciated.