Gtx 670 fps's drop

I bought my new gaming computer 3 months ago, it's a i5-2500k@stock and a gtx 670. I never used to have less than 60 fps's not even when there was big explosion in BF3 I've always played on ultra settings.. But since a few weeks ago, In some moments it goes to 40-45 fps's .. Is it normal that as the pc gets older to see the fps's dropping? It may be normal, but 20fps's looks just too much!