really need help understanding credit card interest rates

I used to be sharp as a tack but lately my brain just cannot grasp certain things. DH and I decided that we were going to start paying off our credit cards. I paid off my "New Balance" on my capital one card last month and was looking forward to seeing a zero balance this month. Instead, I received a statement with a new balance of $13.35 which were finance charges. So I called them up to get my account adjusted. The woman tells me that I have to pay my balance in full two months in a row in order for there to be no interest charge. So I'm like if I pay my card in full the first month, then there should not be anything left to pay the second month. She couldnt explain it to me in a way that I understood and thought it was her so I asked to speak to someone else. I received a supervisor and she told me the exact same thing, that I needed to pay my balance in full two months in a row in order for there to not be an interest charge assessed. I got so frustrated and ****** off that I just cancelled my card. I just printed out my credit card agreement and i just cannot understand it. Can someone please explain how it works so my challenged brain can grasp it.
How can I avoid paying interest charges? Each month you pay your "New Balance" in full, you will have a minimum grace period of 25 days with no interest charge on all new 1) purchases, 2) balance transfers, 3) special purchases and 4) other charges. If you have been paying your account in full with no interest charges applied and you do not pay your next bill in full, prorated interest charges will be assessed. There is no grace period on cash advances, special transfers, or on any new transaction when there is an unpaid balance from a previous bill.

how is the interest charge applied? Interest charges accrue from the 1) date of the transaction, 2) date the transaction is processed or 3) first calendar day of the billing period. Interest charges accrue on every unpaid amount until it is paid in full. this means you may owe interest charges even if you pay the entire "New Balance" one month, but did not do so for the previous month. Unpaid interest charges are added to the proper segment of your Account. however, we reserve the right to not assess interest charges at any time.


I guess I dont understand "interest charges accrue on every unpaid amount until it is paid in full. This means you may owe interest charges even if you pay the entire 'New balance' one month, but did not do so for the previous month." If i DID pay it in full it is no longer unpaid and I shouldnt have an interst charge.

i always thought that all I needed to do was pay my balance in full by the due date so as not to incur a finance charge. It should be noted that my past balance consisted only of purchases. There were no balance transfers or cash advances.

Let me know if you need any additional information because i seriously think I'm losing my marbles