Sheryl Crow just announced she has a brain tumor -- but the singer insists ... there's nothing to worry about.

50-year-old Crow told the Las Vegas Review Journal, "I worried about my memory so much that I went and got an MRI. And I found out I have a brain tumor ... but it's benign, so I don't have to worry about it."

You'll recall -- Sheryl choked on stage last month in Florida, forgetting the lyrics to one of her most famous songs. She said, "I'm 50 what can I say! My brain has gone to s**t!"

It's unclear if the incident was related to her brain tumor.