I heard a loud pop followed by pain in my knee. How important is it to seek medical attention?

Possible Injury. The pop could be a ligament tearing. The pop and pain are a sign of a possible serious injury in your knee. You should have it evaluated to decrease the changes of long-term problems from an injury.
Seek attention now. Loud pops in knees that cause pain (and early swelling) are most worrisome for an acute ACL tear. Other potential causes include meniscal or cartilage injuries, loose bodies and scar tissue. An expeditious medical evaluation is necessary to figure out your specific injury.