Nagging was the first word that came to mind. Bugging has a very similar meaning as well and could be used alternatively.
Pestering, hassling and harassing have similar meanings. Pestering is more of a mild annoyance rather than harassing, which implies a more aggressive manner.
Pestering: "trouble or annoy (someone) with frequent or persistent requests or interruptions".
Harassing: "subject to aggressive pressure or intimidation".
Nagging: "harass (someone) constantly to do something that they are averse to".
When someone is being pestered, they might say "Get off my ass" or "Get out of my ass" or "Get off my case" which all mean, in simple terms, "Leave me alone" or "Stop nagging me".
So the phrase getting on someone's ass could mean nagging them about something, although this typically implies pressuring or urging them to do something. Harassing would be a better definition in this case.