Reasons for excessive exhaust smoke?

This is a recent occurrence, just started a couple start-ups ago. I'm pretty sure it's burning oil but could be wrong. The exhaust is near invisible at first, as normal, but will start getting REAL smokey within 30 seconds of running. I think it's burning oil because I changed the oil last month and the dipstick already shows a large amount gone. The smoke is also thick and gray, can see blue at night when I shine a light on it. I've heard that burning oil is commonly caused by a clogged or bad pcv valve, but as far as I know the valves are just simple check valves and mine has plenty of clearance. My other guess is the piston rings, no idea what kind of condition those or the pistons/cylinders are in. Like I said, recent occurrences, not sure if rings/pistons/cylinders can go south that quickly.