What Causes Recurring Purple Color Swelling In Buttocks?

I am a 49 yr old female and relatively, in good health. I have had reoccurring swelling in my buttocks (crack area). It is bruise like and can vary from a golf ball in size to the size of my palm. Few days later, disappears. Quite painful and purple in color. Suffering for many years. Have not sought medical attention due to embarrassment. Blot clot? Cyst?? Stress? Prolong standing?

Thank you.