If you have low cortisol levels, and go through something stressful. is it a common pattern to feel worse after the event - when the adrenaline dies down? even days later? rather than at the time of the event?

Cortisol levels usually RISE in response to a stressful event - usually within minutes or hours However how you feel does not directly correlate with cortisol level. Other factors like psychological influences may be at play. If concerned, check with an endocrinologist

No not days later unless you have low adrenal reserve. This is checked by giving ACTH. and measuring cortisol. Would be rare at your age. Discuss with your Drs.