I workout at night and sometimes i wake up the next morning feeling nauseous and a burning sensation in between the middle of my chest, why is that??

Reflux: Could be acid reflux. Try to avoid eating anything (or drinking any protein shakes) 2 hours before bedtime. See if that helps, if not then see your doctor for further evaluation.

Possible GERD: It's not necessarily exercise related. It may be that overnight, when you are lying down, that acidic stomach contents is refluxing back up the food pipe(esophagus), and creating irritation, which may present with the sensation you describe. It is very common that acid reflux is worse at night. But review this with your doctor to be sure. Best of luck.