Can I test my cortisol levels if I am taking hc?

No. If you are taking Hydrocortisone you cannot measure a cortisol lvel. You would need to hold your hc before getting blood drawn.
Yes, but. Yes you can have your cortisol level checked but on the condition that you are off hc for at least 48 hrs. Now practically speaking, checking levels in someone who is on cortisol replacement is generally done if the decision for replacement was equivocal and that you have not been exposed to hc treatment for an extended period of time.