After a cardio i notice my cheeks turned blue, why is that?

Here's a short list: Certain drugs, cold exposure, autoimmune conditions, high altitude, smoking, nerve root compression, blood clots, lung infections, cardiac and/or vascular (blood vessel) conditions can give rise to reduced blood flow and oxygen delivery associated with cyanosis (oxygen-poor blood). Absent these conditions, exceeding one's aerobic capacity (pushing too hard) may be the most likely. Take it easier!

SkinYellow-BloodRed: Skin (alive-without much blood) is yellow (protein color). Blood red, light when o2 high, darker (maroon) when o2 lower & looks bluish though yellow skin, called cyanosis. Ex: press palm skin firmly several seconds & closely watch color change after releasing. If exercise is intense your brain shunts blood flow away from facial skin to muscles which worked hard, slower blood flow/bluish may result.