I was coughing up dark yellow phlegm this morning i been having chest congestion few days im 22 and don't smoke what could it be most likely?

Viral infection: By far the most common cause of chest congestion and phlegm of any color (other than blood) is viral upper respiratory tract infection. No treatment other than conservative measures is necessary. Serious signs such as chest pain, shortness of breath, blood in the sputum, wheezing and other abnormal breath sounds need further investigation by your doctor.

Acute bronchitis : A 3 day history of cough which is productive of discolored sputum, along with chest congestion, is likely to be an acute bronchitis. It may have started with an acute upper respiratory process with post-nasal drip, later extending to the lower airway.