Just skipping one period is sometimes normal, but pre-menopausal is on the cards although a bit early.
A raised (rising) FSH & LH level will confirm this but will need to be repeated at a later stage.
pelvis may also be necessary.
The rash that you are describing could be secondary to the sertraline
, so you could either stop this or get a prescription for long-acting antihistamines for 4-6 weeks (fexofenadine