Sheryl Crow -- Cell Phone May Have Caused Brain Tumor

Sheryl Crow believes excessive cell phone use may be to blame for the benign tumor in her brain.

Crow just appeared on Katie Couric's new show and said the tumor is located in her "cell phone area" ... and says, "I do have the theory that it's possible that it's related to [cell phone use]."

Crow admits her theory has not been confirmed by doctors, but says she's convinced because "[in the] early early days when I was promoting my first record, I did hours of phoners on the old archaic cell phones."

The singer says it became difficult to carry on normal conversations ... claiming she felt "mushy" ... and at one point she was convinced she had early onset Alzheimer's.

Later, she learned she had a benign tumor the size of her pinky that's located between her skull and the lining of her brain.