Indexation Amt

Is a landlord allowed to add Indexation Amt to your rent (Indexation du loyer)after you have given the notice of termination ? As per my house contract after 3 years (which i completed in March 2011) of staying the contract becomes null (implies i think it will follow the 9 year contract pattern) and any issues after that (like house termination etc) will be resolved as per standard Belgian contract laws. I have already notified my landlord by registered letter (in english)that i would be vacating his apartment on Oct 31st 2011. Yesterday (2nd Aug and not by registered post, think he just dropped it in my box) i received a letter with the request to pay up the rent + indexation amount , starting Aprl 2011 till Oct 2011. Am i on thin water here ? Thanks for your reponses in advance