Leg pain during leg raises?

Hi,

I am trying to progress onto doing hanging leg raises however I seem to be either feeling this more so in my legs more than my lower abs. 

Sometimes I even get slight pain around the knee area, would you know the potential cause of this and a solution? I tend to feel it less so if I were to do them lying down but as soon as I progress onto the hanging variation I always feel it in my legs.

I am positive my form is correct and I try to minimize swinging and I am correctly bracing my core when attempting to do this exercise.

Thanks

Josh