get behind with (something)

1. To not be as current with some task as one should or ideally would be. I got behind with my schoolwork when I had the flu. I'm sorry everything is such a mess, I've gotten behind with the filing. Mrs. Smith loves to give pop quizzes, so don't get behind with the reading.

2. To not be able to satisfy one's current financial obligations. All of those medical bills have caused me to get behind with the rent. A: "If you'd had more money saved up, you wouldn't have gotten behind with your bills. Just sayin'." B: "Gee, thanks." Because I'm still waiting for three clients to pay me, now I've gotten behind with my own bills.