I am 17 and I'm getting tightness in my chest and back. I also have extreme fatigue. What might it be?

Asthma? Chest tightness can be a symptom of reactive airways, when left untreated can reduce available oxygen and cause fatigue. Other explanations also exist so a visit to your doctor for an in person evaluation would be wise!
Need an exam. There are numerous conditions that can cause these symptoms. In a 17 year old patient the most common cause of chest tightness would be stress or anxiety. The most common cause of fatigue would be depression. However, in order to arrive at the right diagnosis you need to see your physician who will ask you the right questions, examen you and order any needed tests to arrive at the right diagnosis.