I just had my crown done for my molar and my dentist left a small gap on the back of the teeth. Is this normal?

After placing porcelain crown, you must not feel a gap or a ledge! however we need to see your xray and clinical picture to help you further.Ask your dentist about the fit of your crown, and communicate with your dentist to resolve your problem.

Hi:
Unfortunatley without an X-ray and photo it is difficult to give you accurate advice.  Please post photo and X-ray of the area.
Best,
Dr. Maddahi

Your dentist may have left the margin of your crown above the gum on the inside of your tooth so less chance of gingival irritation. Have your dentist check the crown again. Make sure margin is sealed and if he can possibly smooth the area where the crown margin meets the tooth.
 Make sure that there is no ledge and have an x-ray taken of the fit of the crown.