As a result of doing box squats and deficit deads is you tightened down your hip flexors, namely the psoas and the rectus femoris. A simple fix is to do a kneeling quad stretch. To perform this, start on your hands and knees (quadriped). Place your feet with the soles of your shoes vertical on the balls of your feet. Bearing weight on your hands and balls of your feet, slightly raise your knees off the floor and let your weight sink back into your feet. You will feel stretch in the bottom of your feet. Retain this feeling in your feet and bring your knees back down, slightly wider than hip width. Slowly walk your hands back to your thighs and bring your torso up to as close to vertical as you can. The pull you will feel in your quad will be extreme and you will feel like you are going face first to the floor. Now, keeping the tension through your feet, slowly push your hips forward while holding your abdomen in. More than likely the urge to push your belly out will be more than you can control. FYI 80% of the muscle we are stretching,the psoas, attaches to your lumbar spine and pushes your belly out.
To make this a little easier, keep the foot tension and think of moving your butt back over your heels, push your belly out to feel the extreme, now pull your belly button to your spine. Hold that and begin to push you hips forward like you were doing a pelvic thrust. Hold this stretch a minimum of 30 seconds, repeat 3 times. Do this before you go to bed and especially after doing box squats and deficit deads. Your hips are telling you what you did to them.