It is because of not enough lights on it.

To fix it, I read you could re-pot the plant with the soil higher, closer to the leaves, about a half inch under the first leaves.

Or how about, since its still growing like you say, just leave it on its side. That way it will grow side leaves and will get bushy. Either way, put some lights on it so it doesnt stretch any more. And a fan to make the stem strong.