Hoe Can Pain In The Left Buttock Radiating To The Knee After A Fall Be Treated?

The left side of the bottom of my butt impacted on a large rock when I fell backwards on a rocky slope -- 4 weeks ago. Since then the pain has expanded from the bottom of my left butt down the back of my let to the back of the knee. It is still sore and hurts more when I sit for more than 10 minutes. It hurts when I go up stairs. What you you think it is?