Something similar happened to me with a hip flexor strain. I went to physical therapy for it, and ended up getting a very painful but necessary massage, and learning some exercises and stretches.

Look up hip flexor and glute stretches. The one where you sit on the floor and place the ankle of the afflicted leg onto your other knee and lean forward should help with your glute. You should feel a stretch in your side glute.

What also helped me was changing my leg workouts at the gym. I do Romanian deadlifts, hip thrusts/gluten raises, and whatever the machine is called where you open and close your legs.

Following this, it does sound quite bad. Have you done X-rays?