Posture question

[elbowglitter]

So after my run today, I took my bike out for a quick spin. Didn't want to spend too much time in the saddle because a friend and I are biking again tomorrow and the last thing I wanted to do was be incredibly saddle sore.

About ten minutes in, I started to feel a bunch of tension in my upper back, between my shoulder blades. I also felt like I was resting too much on my hands, perhaps, and this was causing the problem? I'm just not sure where my posture is wrong. Though just looking at the image at the top of this page, I wonder if perhaps my back was much too flat.

I do also carry a ton of tension in my back, so that can't help, and I wasn't exactly the most relaxed, as this was the first time I was on the bike all by myself, and I was playing with gears and seeing how everything works.

Does anyone have any idea what I'm doing wrong or how I can fix this? Ten minutes isn't exactly record time to spend on the bike.