Strong stretching pain in my inner thigh along the inside of my knee when doing lateral lunges or doing a split. any clue what the culprit could be?

Pes anserine tendons: Sounds like you are stretching your adductor muscle group along your inner thigh and on the 'inside' ;#40; medial ;#41; aspect of your knee.This would include the gracilus muscle;#40; tendon;#41; as it courses down your leg to attach below your knee near the medial collateral ligament;#40; mcl;#41;with the sartorius and semitendinosis muscles/ tendons;#40;pes anserine group;#41;bursa."splits"stretch the adductors.