Lower back /upper buttocks pain when standing

I've been getting pain in this area, mostly focused on right side for months. It's a dull ache. I only get it when standing. It comes after a few mins of standing in static position. I can run 5km with no pain. I get no pain when lying down and minimal when walking. Pain does not go down the leg. I've been told by physio and massage therapist that I'm very tight. Particularly on right side. I've tried physio to strengthen glutes and core. I've been given stretches but I'm not making gains with flexibility. I've had massage and acupuncture also.

Anyone have this issue before? Any tips?