I recently found black pigment on inside of my cheek where my wisdom teeth is and also black pigmentations in roof of my mouth. when i google searched it showed melanoma and i m v much stressed?

Don't stress. Not sure what your risk factors are (smoking, chewing tobacco, alcohol), but internet searches are not the way to go. You'll need to make an appointment with a local dentist or oral who can actually examine you and biopsy the lesions if it is necessary.

A MELANOMA IS EXTREMELY RARE IN ORAL CAVITY,ONLY 2% OCCUR IN MOUTH.THE PIGMENT COULD BE NORMAL FOR YOU OR FROM MEDICATION. SEE AN ORAL SURGEON FOR A CONSULT.