Could my GERD be aggravating my asthma and causing pain across the middle of my back?

GERD and asthma. Yes - gerd can aggravate asthma. There appear to be two mechanisms at least. One is reflex stimulation at the end of the esophagus triggering cough; the other is the possibility of very small amounts of reflux going down the airway. In either event, this should be addressed sooner rather than later.
Yes. Yes and less likely. Gerd is a common cause of asthma exacerbations and poor asthma control. Esophageal injury from gerd or esophageal spasm can cause upper back pain but this is uncommon. You might consider a gallbladder evaluation for the back pain.