Can a bank add a months payment to a repossession even if that months bill Isnt due?

So long story short my boyfriend and I have gone through some hardships in that last year, from getting laid off. We have fallen behind on our truck payments but have gotten caught back up and had a arrangement to pay the last payment to be completely caught up this friday. We have our truck loan with Compass bank and today we called to get the balance of what was due friday, come to find out they have placed our truck in repossession. Apparently the payment we made previously was one day late cuz the bank entered it to late in the night for them to get it same day. Anyways as angry as I am I understand the bank has the right to put our vehicle up for repossession, now my question comes into play. In order for us to take it out of repossession we have to pay a fee to get the notice off plus the balance that was still due, not a big problem except the bank is saying that the balance that is due includes next months payment plus what we were planning on paying friday. How does that work our payment isn't due till the 10th of the month, why do we have to pay next months payment when its not even up to be paid yet? Now Im pretty ticked off cuz how is that at all legal? Can someone please help i have no idea what Ima do since i don't have that kinda cash now!