Pain in lower back

Hey girls I keep getting sharp pain in my lower back and even weird on my butt cheeks and down abit 
Is it just Nerv pain could the baby be sitting towards my back? I'm 12 weeks tomoz this is my first 
It only happens if I do to much in one day it's like my body just can't handle it but when u have a sister that just wants me todo everything for her and her son and drive for her so she can have a beer if I don't do it she makes me feel bad and uses my nephew against me even he tells her off he is only 6