What is crypitc pregnancy ?

Hey ladies. I had a very short period this month. It was 2 days of spotting, i dont have any underlying condition that would cause this, im bloated but have no other symptoms. Im currently cd 21 and havent had a positive opk and people keep asking me if im having a cryptic pregnancy. All test have been negative. Could this be a cryptic pregnancy? What even is a cryptic pregnancy? Idek.