It could be abs. If you are overweight that can contribute also. Can you do a bodyweight squat without ballance issues, going all the way down? If not you're not ready for lunges. If so, you should be ready soon. I would work on abs a bit more. Work your quads/glutes with leg press. When you are ready to go to lunges start with the one where you are standing (with or without barbell), then lunge on 1 leg, then back, then the other leg then back. Get that one down first before you go to the walking dumbbell one where you look like you are trying out for the minisry of silly walks.
I thought those unfamiliar with it should have a visual. :)