Intense sensation when I stretch my back?

I have a bad back, so I started stretching it 2 days ago. I sit down cross legged, and lean foreword, and try to reach my head as close to the ground as possible, almost as if I'm doing the butterfly. I place my hands on the back of my head. A couple seconds in, my back, my quads, or my abs (it changes every time) will start to burn, but it's a really good burn. when I sit back up, I feel a really nice sensation rush through my body, almost orgasmic. Is it because my muscles are so tight and they're finally getting some movement? I just want to make sure this is healthy and I'm not straining my back.