Have you recently increased the amount of time you spend cycling (e.g., because you didn't ride much during the winter)?
If yes, did you ride a lot last spring/summer/fall with no back problems?
If yes, you can probably safely ignore all the other well-meaning advice above. The cure is to gradual increase the amount of time for each ride until you're back where you were last year.
My credentials: 70-year-old ex-racer who started racing in 1964. Still do mainly training-effort rides. Never stretched, never lifted weights, never worked on my "core," etc. By the time I'm up to doing 16 hours of riding per week, any minor discomfort I experience during the transition to warm weather has mostly disappeared.
Just ride.