Other than a medical issue, I have some observations:
This may seem like an oversimplification - but, the mattress that you sleep on, does it give you the kind of support that allows you a good night sleep? A poor mattress can flex the spin, especially at the base, or lumbar, and pinch nerves and flex muscles.
Before all this came about, were you trying to change your pitching movement, alter your delivery posture, or copy some pitcher’s style?
By not properly preparing your body for the practice and field performance, especially in cold weather, can have far reaching problems with the lower back.
After a workout, especially in cold weather, not hitting a hot shower and relaxing those tense muscles in the back and lumbar area and result in stiffness and discomfort that can build.
And finally, coming off the off-season takes time, gradual exposure to movement and stress load demands on the entire body. Doing long toss by bending back and launching toss after toss, without gradually working into it can produce the symptoms that you describe.