I have seen this before!! When you open the rear of the engine and you will prolly find everything might look ok, other than some bits of metal. If that is the case, then you prolly have experienced the retaining wire that holds the wrist pin in the piston has come loose and wedged between the sleeve and piston. Look for a deep verticle groove in the sleeve wall. If this is true you will have to carefully work the piston to bottom dead center so you can work the sleeve out of the block to replace it and the piston. Granted its a fluke, but I have seen those symptoms before!!

Be very careful, working the piston down, you can bend parts.

Let us know what you find!!!! I might have something to help you remove the piston and sleeve..

Good Luck!!