1/2" pipe fittings don't fit old pipe- PLEASE HELP

So, I'm replacing a leaky supply line in my kitchen sink; it's 1/2 copper pipe, I have the new stubs sweated together, the old pipe cut and cleaned, everything's ready to go, except... the coupling won't fit!

I measured a piece of the old pipe I cut out, and the diameter is just slightly bigger than the new pipe; about one wall thickness extra, barely (but definitely) too big for the coupling to fit over.

Here's the really weird part: I un-sweated the old pipe from the elbow it had, thinking I could use the elbow on the cut I'd made, and now I have a 6" length of the old pipe. The end that was sweated into the elbow fits fine into a 1/2" coupling or elbow, but the other end (where I made the cut) is too big! Is it possible the pipe swelled over time? It is the hot water supply, if that would make a difference, and it's probably 50 years old...

Has anyone here encountered this problem before, and more importantly, does anyone know a work-around? I'm calling the specialty plumbing supply tomorrow (it's 11 pm and I've given up on having water tonight, or most of the day tomorrow at this point), and I'd like to know what to ask for...

Thanks,
Cary