2004 Pontiac Grand Prix Engine burning oil

I change my oil every 3000 miles without burning or losing any oil. After my last oil change, the information center came on at 2500 miles and said low oil warning. I checked the dip stick and found I was 1 1/2 quarts low. I checked for leaks and found none. I filled to the proper level and checked after each fill up. I am getting 900 miles to a quart. What could be the cause of this problem.