My cousin has had this problem and she ended up going to the doctors together it checked out. They told her that her back had a severe arch in it, must be because your spine is connected to your butt I guess. Hope this helps
1) Coccydynia- it can present with what is referred to as taut bands which are strongly contracted bands of muscle fibers that are present within your normal muscles. They form contraction knots or myofascial trigger points-areas that if pressed causes pain. These taut muscles can occur in the gluteus maximus and adductor magnus muscles. The adductor magnus muscle trigger points when pressed can cause the pain to extend into the thighs and also the pelvic floor, so this might explain your occasional vaginal pain as well. Also this can cause painful intercourse as well.
Diagnose is through rectal examine of the pelvic floor to locate the compression knots and trigger points.
Treatment can consist of compression, injections with Procaine, dry needling, or manual stretching of the particular muscles.
2)Gluteus maximus syndrome- with this you have spasm generally on one side. Also with this syndrome the pain will disappear when sitting and returns when you are lower yourself to the position, standing, or squatting.
Diagnoses is done by- Monitoring for the pain upon lowering, rising, and squatting and disappearance in pain upon sitting.
Treatment consists of 3-5 manual treatments of the back and buttock.
3)Piriformis syndrome-the piriformis muscles runs from the spine to each thighbone. This generally causes low back and pain that can radiate down the buttock and into the leg. Pain is often worse upon sitting.
Diagnoses by particular symptoms and attempts to stretch this muscle. To check for this lie in bed on your stomach, bend your knees 90 degrees, and attempt to spread apart your feet while keeping your knees together and bent. If this causes you pain this could be what you have.
Treatment is by injections of corticosteriods, massage therapy, or physical therapy to stretch the muscle.
I HOPE SHE FEELS BETTER SOON. I DO SUGGEST GOING TO THE DR FOR FUTURE REFERENCE.