Not all muslin is created equal. Some has a really large amount of shrinkage... and some never even gets close to lying flat without alot of steam and a good hot iron. Take a piece, measure it , and launder it prior to use.
I use muslin alot for backings and have come to know which ones are better, in your case you don't know much about the manfuacture or even what year it was produced ... TEST!