Ok, I'd start with the obvious ones;
test for brakes dragging by undoing both brakes, and see if it's any better
if no, test the wheels for drag in the hubs, spinning them in your hand ought to do
if still no, test the bottom bracket for play, see if you can't get the cranks to move laterally
if still nothing, my guess would be your saddle position has changed. This happened to me once, and I tore apart the bike before I realized my saddle was a good inch lower than it was before.