Wasn't able to do the "one leg"exercises in proper form. Core to weak?

About two hours after the exercises, there is a pain in the lower back, left side ( IS?) when rotating my upper body to the right or doing a sudden uncontrolled movement. I think it's from the lunge with knee drive exercise.

Struggled with Rotation today. Could only move the half range until hips startet to make the move.

Hi, I 'm week one, day three now. Besides the movements from the program, can i do my usual things like working and riding mountainbike? In a softer way?