What Causes Fluid Filled Bumps On Buttocks?

II get large purple bumps on my bum actually mire like lumps they carry a greenish fluid along with translucent at times and usually have blood to. They are hard but are relatively easy to pop but it hurts alot they leave a small hole and a dark mark that dint seem t go away and they seem to occur getting I menstuarate can y help? What Di I do? What us it from?