nag at (one)
To lecture or reprimand one in an annoying, pestering manner, especially about something one considers minor, trivial, or inconsequential. Would you stop nagging me already? I'll do the dishes later! Look, I don't like having to nag you about this, but those reports really need to be filed today.
nag at someone (about someone or something)
to pester someone about someone or something. Don't keep nagging at me about her. Stop nagging at me!