Its because of of sudden overuse and fatigue. Any activity should be started slowly and intensity & frequency should be increased gradually. Your VMO muscle has started paining.
Next Steps
Go to a physiotherapist.. Ultrasonic therapy +IFT +Heating therapy +some strengthening exercises will be helpful for sure.
Health Tips
At home.. Rest +hot water fomentation +pain killer gel +no long standing /walking +avoid running for a week+use knee cap while running +no cold exposure.