Single tooth pain with Invisalign. Wondering if I have a cavity, or a cracked tooth, or if it's just the aligners causing pain.

At each stage of treatment you may find varied degrees of pressure exerted onto certain teeth.  This is normal and varies based on the particular aligner.  While people may have different pain tolerance levels, most Invisalign patients do not describe their pain as excruciating.  It is best to see your orthodontist or general dentist for the pain if it is indeed excruciating.  

Sometimes during the treatment, some teeth may feel more discomfort than the others. I would suggest to speak to your treating orthodontist and mention the discomfort and pain you experiencing. Excruciating pain is not normal with invisalign treatment. Usually the discomfort is mild to moderate.