Do you have any paperwork that covers what you were paying. Some places have you pay first and last, some places have you pay a security deposit and a "move in fee" that has nothing to do with rent.

I personally had an apartment where I paid first, last, and a security deposit. I lived there for several years and ownership of the property changed while I lived there. When I moved out, the new company tried to keep my security deposit because I didn't pay my last month rent. I had to spend several weeks going back and forth with them over this. I had a friend who was a lawyer contact them as if I were taking them to court and they immediately paid up. I'm pretty confident that they knew I paid my last month rent when I first moved in but thought they could just bully me into letting them keep my security deposit.

So if you have anything that shows what you paid when moving in, you should keep that on hand as long as you live somewhere. They might be trying to fleece you, you might have misunderstood what you were paying, it might be an honest mistake on their end. Unfortunately I don't think anyone here can answer the question for you because we wouldn't know what you actually paid for