Post a pix if you can w/ a size reference. Also, what breed is it?
I've had some look like what you described and it's just the way that breed grows. I have a Sativa breed right now that is about 30" tall and the stems half way up are about 1/8", and the top fourth are about 1/16". I'm surprised it can support itself. Also, all the males I've grown have been a bit thinner and taller than the same breed female.
Like motoracer said, correct light height will control growth. When light on the stem is weak it allows the stem to grow, enough light on the stem stops growth. That's why a plant leans to the light or follows the sun, the light side isn't growing and the dark side is. If not enough light all around then it grows all around at full speed which is straight up.
A fan or manual movement will toughen up a plants stem so it will be strong enough to support height later, and more importantly support buds which will grow weight faster than the plant can grow stronger to support it. So strengthen your plant now before it buds or you'll need to use sticks/string later. In nature the breeze is enough, so try to simulate. Even a little boxer fan from a computer, old power supply or elect device can be used.
