Will I continually get a periodic rate finance charge?

I have a Bank of America credit card. We just paid it off completely, last month.woohoo!
I got a statement this month, and I owe them money for a “periodic rate finance charge”.
I looked it up and their site says this
What is a periodic rate?
The periodic rate is a rate of finance charge that may be imposed by a creditor on a balance for a day, week, month, or any subdivision of a year.

I’m confused. I paid it off, what are they charging me for? And will I keep getting it each month, to continue to have a credit card with them?
I’d rather not get rid of the card, because it’s our “oh shit, emergency!” card, plus, getting a credit card at the rate I had will be impossible now.
Anyone able to explain it to me in a way I might understand?