Capital One Credit Card Complaint

Capital One Credit card department,

APR or interest rate Nevada

I was charged interest on purchases for a credit card that had been paid in full the prior month. When I called to find out why I was charged interest, I was told that the interest was residual interest and unless I paid my account in full 2 months in a row, there would be residual interest. I asked them to explain to me what purchases had residual interest and they could not explain it.