Aww mama, three weeks delayed is definitely something to get checked out right away, I know that nervous feeling when your period is playing hide-and-seek! Thick endometrium with a corpus luteum cyst can happen, pero kailangan talaga ng ultrasound at proper follow-up with your OB to see kung ano exactly ang nangyayari, especially kung may pregnancy possibility pa. Please don't wait too long on this one kasi alam mo kung paano mag-overthink tayo, better to have peace of mind from the doc agad!