Our initial assessment is likely caused by inactive muscle groups that are working since you started following the Unlock Your Hip Flexors program. Make sure you are aware of your proper body positioning when you do the exercise as this so important.
Here is a short blog post about what you need to focus on when doing these exercises: http://exercisesforinjuries.com/3-tweaks-to-help-protect-your-back-from-back-pain/
Tight hip flexors can also affect the pain in your lower back. When you unlock your hip flexors to the proper length, it will make it easier for you to keep your spine in a neutral position. It will literally change your life!
Do the routine every day but let pain be your guide! Most importantly remember, our bodies were meant to move!