Practicing my left split, i went down & think i locked my leg to quickly and there was a little pop at my knee, hurts to stretch, barely hurts to lock?

Knee Pop: Due to the fact that you heard a pop and you are having pain, i would have your pediatrician take a look at you. Most likely, this will not be painful in the am, but better safe than sorry, as you seem to be involved in athletics.

Hamstring: You may have strained/torn part of one of your hamstring muscles . Stains take about 2-6 weeks to heal. See orhtpedic doc if symptoms persist or wprsen.