Western blot band for Erk and phopho(p)-Erk

I want to know Erk activity in my animal model - sepsis.
I isolated it from liver, homogenized it, separated it by SDS-PAGE, and detected it, but I just found one band for Erk.
Procedure was the following:
1. block for 1h @ room temperature (RT) with 5% BSA
2. 1:1000 Erk or p-Erk (cell signalling) in 5% BSA, for 1h @ RT and O/N @ 4'C
3. 1:5000 2nd Ab, for 1h @ RT

How can I get two bands for Erk?
Because most papers show two bands for Erk, is there a problem that mine just shows one band for Erk?