depends on the type of pain. what did you do? plyos or weights?

if you did weights, you prolly just have muscle soreness from working your lower back muscles really the first time in your life. thats normal, even though its not good. you should start thinking about post-workout nutrition.

if you did plyos, you probably strained something. re-organize your dynamic warm-up.

in either way, stretch your lower back, a lot. for stretches see www.exrx.net or www.bodybuilding.com