You know what it is most likely, you must've fallen or something and then you started walking a certain way that just completely ruins the alignment of your posterior chain and back. Thats what's happened to me, you should try stretching your hamstrings, hip flexors, and strengthening the back. I hope this helped I can only speak from personal experiences. If this doesn't help you should consult a doctor and they'll probably recommend you to a physical therapist and they'll tell you whats wrong with your back.