I get the same thing, RFF and tight left hip flexors. Mine gets worse as the season continues. Sometimes I'll wake up in the middle of the night in a fetal position and can't extend my hip. When doing the suggested stretches for the left hip, flex the left glute. The glute is the opposing muscle and flexing it will give the hip flexor a better stretch. Also, get in the gym and work those glutes! Squats, deadlifts, RDLs, hip bridges, hip thrusts, etc. I'm a personal trainer and spend a lot of my time working on this issues with my clients that are desk jockeys.