After about 2 weeks was when I switched to alternating with power cleans, and it was about the time I was pulling 300lb. This is a good change.
For sure check your form. I remember early on experiencing a lot of low back fatigue as well. I attributed it mostly to just having a weak under-used low back. Every other day hitting it with Squats and DL in consecutive workouts, gives it a lot of use, and makes it strong. It was my experience that this will go away before much longer unless it's caused by a technique/form problem.
Keep up the good work!