Acute Upper-Back Pain.

About an hour ago I had an acute upper back pain. When I move or sit in a certain way it hurts like hell. Any idea what I did or what it could be?

You may have gained muscle in your back from last time you went gym, So give it a day or two to rest then if it still consist's then maybe it is not muscle

You might have pulled a muscle? Have you been lifting anything too heavy, or doing any weird stretches?

Also if you work-out which i think you do, then it could just be muscle soreness ?

You could have done a number of different things, If you have sharp shooting pains when you sit in a certain way then it could be something to do with a nerve. Possibly being trapped in the wrong position... this will fix itself when you move in a certain way it will release it.
The other thing is you could have stretched the muscle, you are very unlikely to have torn it or anything. If it is from working out then your muscles should ache and be sore after every workout... This means you have worked them in the correct way and to an extent to actually gain something from it.

If it continues you should go see a doctor as it could be something to do with your spinal cord! Doubt it very much though.