It sounds like it could be your sciatic nerve. If that nerve is pinched by either a bulging/herniated disc or muscle spasm, it can cause pain in your rear end and down the back of your leg.
I've had two herniated discs (resulting in two back surgeries), and I had horrible pain down the back of my rear and leg. It was pretty constant though, and I got to where I couldn't walk.
After my 2nd back surgery, I tore my piriformis muscle (a tiny little muscle on each side of your bottom). This muscle spasms frequently, pinches my sciatic nerve, and causes the same type of pain the herniated discs did. The difference is that it goes gets better with movement and massage, whereas the herniated disc pain got more intense with movement.
The next time it happens, you might try massaging the areas to the left or right of your tailbone (and up a little bit) to see if maybe your piriformis muscle is spasming and pinching your sciatic nerve. This muscle stays tender and sore on me most of the time. Heat also helps me when it is in a spasm.
"There is no charm equal to tenderness of heart." - Jane Austen
Fibromyalgia, 2 back surgeries, Meniere's Disease, 30+ kidney stones, GERD, IBS, Asthma, Allergies, Endometriosis, Heart Arrythmia, Myofascial Pain, TMJ.