What Causes Burning Sensation In Thighs And Buttocks?

Hi, For a while now it has been a bit hard for me to sit on my buttocks. Before my legs used to be the problem. They used to feel like their getting tight and I felt a little burning sensation. But now the pain has grown up to my buttocks, and its like my skin is ripped or something and someone is sticking me with a needle, all throughout the back. I really don t understand why it feels like its getting tighter. This is making me very irritable because i can t sit for long without having to move my butt around from time to time, and my legs hurt and feel weak. What should I do?