Creatine itself does not cause lower back stiffness. What it does is pull water into muscle, so if your fluid intake is low you get tight, cramp-like muscles - and with HLA-B27, ESR,
CRP and RF all normal, inflammatory back disease is effectively ruled out. Your pattern sounds mechanical: tight hamstrings/glutes and weak core, worsened by dehydration. Drink 3-3.5 litres water daily while on creatine, split the dose, and do daily hamstring plus hip flexor stretching and core work.
Next Steps
Try 2 weeks of extra hydration and stretching. If stiffness still returns, or you get morning stiffness beyond 30 minutes, message me for a consult on Practo and we will look at
vitamin D and posture in detail.