What could a small black spot in my upper mouth above the gums be. i smoke. could it be a stain from smoking or cancer? it wasn't there 4 months ago.

See a doctor: Changes in color in the mouth, especially in a smoker can be a sign of cancer. See a dentist or ear, nose and throat physician as soon as possible to have it evaluated.

Many things: You can get a freckle (melanotic macule), a mark from filling metal if you have dental fillings (amalgam tattoo), or something more worrisome. You should visit a dentist or oral surgeon for an examination and possible biopsy.