Ah, okay. Then it sounds like you got hit with The Bug. The collision areas that reveal the tunnels tends to be what triggers it (though is not the direct cause). You're not supposed to be able to return to full sized while inside the tunnels. And of course, while you're small, you shouldn't be able to open normal sized doors. Something about the way the shrink layer is implemented is causing the lock on the size mechanic to trigger in reverse; instead of locking you out of returning to full size when the tunnel is entered, that lock is being triggered on exit instead. And of course, if you return to full size while in the tunnel, you get stuck.
I'll be rebuilding this soon, and hopefully implementing the shrink mechanic in a way it won't bugger up so quickly.