If you mean your erectors, that's a bit odd but not really out of the ordinary. Honestly if you don't get a lower back pump/some DOMS in your lower back a day later, I'd probably question how heavy your deadlifts were in the first place. Your erectors are always working for 9/10ths of the deadlift, stabilizing your lumbar curve throughout almost the entire movement. How much were you pulling? Once I get above 3 plates, I can feel the tension/pressure on my lumbar curve if it starts to round forward, so I know when my form is breaking down and know when to back off a bit if I'm too tired.