Gluteal tendon tear?

I have had pain at the back of my pelvis, right side, on and off for several years. It's just above the hip joint. Originally it started after doing a lot of driving in a fixed position over a couple of weeks.

Recently it has become much worse. It tends not to hurt during the day, or when I am standing up, but it starts hurting in the evening particularly if I sit in a relaxed position on a sofa. It then becomes particularly bad lying down at night - sometimes i cannot find a single position that does not hurt. The pain sometimes radiates down the outer thigh and the front of the shin.

The doctor sent me to a Chiro who just says its a "strain" without being any more specific. It has got slightly better but I think that may have happened anyway.

Looking on the web I wonder if it could be a tear of the gluteal tendon? If not, what else?