Back keeps locking up?

I have had a disk a slight disk protrusion last year but this week my lower back has been bad when I go from sitting to standing and standing to sitting or lying down to standing etc.. It feels like my lower back is locking up and it hurts to stand straight up so I try to stretch it off but it hurts to move. The pain is helped by heat, I'm a teenager.. And I don't know if it helps but the disk protrusion was L5/S1