My apartment landlord has made a mistake...do I have to pay for it??

It is now April and my apt. has told me I owe $653 that wasn't paid in December. I never was told this, I've paid rent since and never received a letter of an unpaid balance. I know it's something they did cause on my payment ledger it shows payments that aren't accurate like a $1000 money order that I never made, I always pay rent in small portions because I have roommates. And why would they wait till now to say we owe it...when I was late on my rent they gave me a 3day notice and I paid it. So why would they just now say something about 653 from December. What should I do!! I'm going crazy!