My guess is you are starting to overstride and maybe not leaning enough. Two rules to keep in mind are 1) don’t step past your hips and 2) don’t step in front of your knee. Also, you may need to relax more to allow some pelvic rotation. Let the ground ‘grab’ the back foot, which in turn pulls the leg and then the hip back. This lets you stride behind as opposed to reaching out front.
Hope that helps!
Certified Chi Running and Chi Walking Instructor