I am 21yr old and my periods has been delayed by 19 days. what could be the possible reasons for this ? should i consult a doctor ?

Period delay: You may be pregnant so either do a pregnancy test or go to the doctor and have him check you out.

Not needed yet: The most common reason for a late or missed period (besides pregnancy) is a late or missed ovulation. This isn't strictly "normal" but it's extremely common. You can see a doctor anytime you like, but until/unless you miss a second period you might wait. If you're sexually active a pregnancy can be done now.