Being charged rent that I don't owe?

My girlfriend and I work at night and sleep during the day. Recently, we were awakened by someone pounding on the door of our apartment. Half asleep, I stumbled to the window to see who it was. I did not recognize the person and headed to the door. As I went to open the door, a man let himself in with a key!! He was from the real estate company we rented from and was coming in our apartment uming we were not at home. He said that they umed we had moved because "we haven't heard from you". Our rent was due at the time, but not late. There was no reason why they should expect to "hear from" us. We called the real estate company, but they offered no explanation as to why they would send a stranger into our home. We decided to move. We took our rent money and got another apartment, called the real estate company, and told them we were vacating. Within a few days we were gone. This occurred in early August. Now they have sent a registered letter claiming we owe them money.