John, as one who sometimes hung out with Bert when we were both very young, I can tell you he's been stretching his fingers around crazy chord positions for most of his life, which is to say, since he had the fluidity of youth on his side. It is harder to train older fingers.

Also, I'd be curious to know if you hadn't played (or hadn't played consistently) for an extended period of time before you attacked Bert's tune. Going from resting to playing full out could cause injury.

And finally, though you've been given lots of great advice here, if you are a gigging musician, you will want to seek help from a sports medecine specialist, someone who can help you "play through the pain" so you don't have to give up your livelihood.