It shouldn't be that way, usually as long as you put in some kind of ranked matches, you'll be fine. If you don't play for about 2 weeks, you'll get reset.
You are probably just another victim of the growing list of rank resets, welcome to the club!
On a side note, teams ranking seem to be permenant. H2H resets more than FFA, but there are still random resets in FFA too.
It should also be not for other readers that PS3 is not this way. Rankings don't get reset on PS3 for inactivity, but on Xbox, if you don't play for a while, you'll get reset.