I think your rear wheel must have slipped. Chains don't 'stretch', they wear. To quote Mr Brown:

"This is not actually how chains elongate. The major cause of chain "stretch" is wearing away of the metal where the rivet rotates inside of the bushing (or the "bushing" part of the inside plate) as the chain links flex and straighten as the chain goes onto and off of the sprockets."

So moving to a half link chain would not really solve anything.