If you had no prior outstanding balance when you paid early, then all of that payment should have been applied to your next month's rent---so long as you have receipts you would have a strong defense to an eviction process. My guess is that there is a fact or two missing that would make the scenario understandable.
(1) gather together all notices, emails, letters or any other documents related to your tenancy.
(2) continue to pay your rent on time and in full---ask in full for a document in writing that explains the $280 demand
(3) contact a landlord tenant attorney in your area (call the county bar association and request a referral or use the Avvo.com search option to find one). Some helpful links are provided below:
http://www.courtreference.com/Courts-Online.php?court=5278 for legal services options
I am licensed to practice law only in the Commonwealth of Virginia, I try to provide you with sound guidance--but not legal advice. Answering some of your questions does not create an attorney-client relationship and I am not obligated to respond to subsequent questions or emails. If my advice helps you get on the right track, select the 'up' check button on the Avvo.com website. Best of Luck.
In addition to Mr. Rafter's comprehensive answer, you may wish to mail a letter via registered or certified mail to your landlord noting your version of the events and providing proof of payment (i.e., copy of the cancelled check).
This information is for general information purposes only. Nothing stated should be taken as legal advice for any individual case or situation. This information is not intended to create, and receipt or viewing does not constitute, an attorney-client relationship.