Am i liable for an extra 5 days rent after my lease ends?

So basically my lease is up at the end of this month and i thought this was the end of it. So today i get a call from the office asking me whether or not i was going to renew, i said no. They say that because i did not give them 30 days i owe them an extra 5 days rent. How is this possible? I am not on a month to month basis, my lease began sep 1st last year and ends this year aug 31st. And no i did not receive a paper asking me whether i intended to vacate, only a letter stating that my lease is up soon and i should call the office should i feel the urge to renew my lease for less.