How did my rear freewheel become incredibly tight overnight? I got a flat yesterday morning after a normal ride, walked my bike inside the house and propped it upside-down (like this) overnight.  This morning, I changed the flat and put on some brand new tires I bought last week.

When I take it out for a ride, I find that the freewheel is nearly locked up!  I can forcibly move it by dragging on the pedals and bunching-up the chain, but it's not easy.  I've tried adjusting the tension on my quick-release, but that doesn't seem to make a difference.

The freewheel on my SRAM S27 has always been tight (to the point of putting some slack in the chain when back-pedaling), but now I can't even coast!  How did this happen, and what should I do about it?

Edit - the next day:

I made my adjustments on Tuesday morning as I said in the comments, but after only a 15 mile ride the freehub was completely tightened back up again!

After looking into it a bit more carefully, I found a service manual for my wheels with some nice pictures.  Here's what I adjusted on Tuesday morning:



I apparently loosened the "end cap" from the threading on the axle.  Here's another pic of the whole assembly:



Unfortunately because it tightened right back up that obviously wasn't a very robust solution.
I decided to put up another answer because while Daniel's hunch and Billy's excellent answer get at the gist of the solution, I did find some specifics that might help someone else (though perhaps only a SRAM S27 owner).

First of all, the service manual is pretty helpful.

In my case, the "end cap" was tighening on its own during rides.  Normally, this little set screw is supposed to keep the end cap from threading or un-threading (ie. you get the adjustment just right, apply the set screw, and it should all just stay put).



The set screw actually applies pressure to a rubber o-ring, which in turn presses against the threads on the axle to keep the end cap from turning.

On my wheel, that o-ring had actually partially disintegrated where the set screw contacted it.  So instead of pressing a healthy o-ring against the threads, it just sort of mashed some rubber particulate in there.

The right solution to this problem is probably to get a new o-ring.  The route I took, however, was to rotate the o-ring about 90&deg; so the set screw was once again above a healthy bit of rubber.  I then re-adjusted the end cap tension and re-applied the set screw.

Hopefully that keeps me going for a while without tightening up again.  Thanks for all the help, everyone!