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What would cause the chain to get tight. I had the rear wheel off a month ago to change the rear sprocket. Put everything back together. Re-aligned the back wheel. Adjusted the chain tension per the manual and everything seemed good. Last Saturday went on a 550 mile ride and heard a growling which I thought was from the pipes. Last night checked the chain and it was tight. Any ideas. Thanks

You're saying the tension got tighter on the chain or that it is stiffer than when installed?

Cahins stretch, cahins get stiff from lack of lube or from rust and adjusters can loosen causing the chain to get loose but I can't think of a reason for it becoming more tensioned... hmmm, I'll have to give it more thought I guess as I've never had or heard of anyone having that problem.

Chains can wear unevenly, did you check one spot that happened to be tight? Spin the wheel and check the chain in 4-5 other spots. Find the tightest spot and adjust the chain per the manual.
I kinda like the idea of replacing chains and sprockets together. If your chain is much older and stretched a bit before installing the new sprocket they can have a hard time mating together. Just my .02 cents.

Checked the chain. Tight. Rode home 2 miles. Checked the chain. Tight.

I was going to post this exact same thing, but I figured I must be wrong.
Checked my chain last weekend, and it was a little slack, so I loosend the axel nut and put a turn on the chain adjusters. Checked the tension (about an inch of play at the tightest spot), and tighened up the axel nut (don't forget the cotter pin).
So two days later I'm cleaning the chain (I need a life), and it's pretty tight...maybe 1/2 inch of play in the tightest spot. Can't explain it, just thought I'd share.