Based on what you said I would **guess** your back has deconditioned or you have a strength imbalance.

My physical therapist says it's common for cyclist to have back (and knee) problems due to the inperfect nature of cycling as an exercise. It's great for your quads and heart but if you don't strengthen you back (major stablizer) to keep up with your leg strength, you end up hurt.

I started training my entire body (rather than just legs) and I've been injury (knock on wood) free ever since.