pains when streching

I have been doing a dvd work out for a few weeks now and there is one strech (slide plank.) where u have to have one knee on the floor lean on hand and strech through the leg,

On the dvd she keeps saying feel the strech, but i am not feeling anything, i can do the splits and quite flexable, am i doing this one wrong,

When i do the core work out i feel like things are going to snap they get so tight, and the burn so i know its working,

Help please xx