Needle feeling in my back when It hits the sun?

It's not the tingling or numb or ticklish feeling...like the kind some people get in there legs.. but it feeling like thousands and thousands of needles stabbing me again and again it sends a wave through my back and It hurts so much I sometimes drop to the ground, but it only happens when I'm in the sun, and only on my back. I've been to the doctor, they say I'm perfectly healthy.