Your plant is on the taller side for an autoflower but nothing to worry about as long as its healthy. The causes for this is probably some stretching if you have been moving the light up frequently but it might just be that particular plant.
I recommend that you add some support if the middle stem looks thin/weak because as the buds start fattening up you don't want the main cola breaking off. Happened to me once, not the funnest of experiences.
Good luck on your grow and hope you get a good smoke off those top cola buds