What should I do ???!!!!! ?

Moved into my apartment had a 1 year lease. 4 months prior to the end of the lease I received offers on my door for releasing. I never responded to those offers that had “expiration dates” with each deal. 

On the last day of my lease I was completely moved out, had my utilities disconnected, returned equipment to my cable/internet provider (although the cable/internet services were paid for through the apartment with my rent as a part of the “tech package” that was to end when the lease ended) 

I never made contact with the apartment management property I just assumed it was implied I needed to be out by the end of the lease and and that clearly I wasn’t releasing if I didn’t sign anything and you didn’t hear from me.

I moved out 3 and a half months ago now, and I steadily receive a monthly bill from my apartment complex they still charge me rent each month on the first, a late fee for non payment, the tech package fee, and utilities. I’m totally lost as to why and how. Also in the emailed billing ledgers to me state me lease start date now as the end date of my actual lease, and it doesn’t have a date filled in at all for the lease end date. There is also a charge each month that says market rent. 

I decided to visit the apartment last week to see if they have rented it out. My apartment is completely empty still in the exact condition I left it in literally down to an old news paper I dropped on the welcome mat that’s still in place. What do I do? Can they do this ??