Lower back paid during deadlifts and squats - ruined FB Booty for me!

Hi everyone, I'm hoping someone can give me some advice.

I just finished FB Booty but really felt I didn't get as much out of it as I should have because every time I do a few reps of squats or deadlifts I get this dull achey feeling in my lower back.

I feel like my form is ok, my back is flat for the squats, I'm pushing out hips and glutes first, same with dead lift, flat back and bracing core.

I don't THINK I'm lifting too heavy (I used Powerblock dumbells - only deadlift at 11kg per hand and squats max 8kg per hand at the moment) any suggestions?