What Causes Pain In Tailbone And Waist While Bending?

Hi, I am a female in my early 20s. I have had a mild pain in my tailbone when i bend over for just over a year. It has been getting slightly worse over the past month or so. It is a very sharp pain when i bend over at the waist, when i drop my head forward, or when i sit for a long period of time. Any ideas of what is causing it?