Flannel has a reputation for shrinking. Especially the lower grade ones.
WASH IT before using it.
I even soak my fabrics in hot water before using them. for about half an hour - or longer - if I get sidetracked.
I can see some of you cringing out there - but I just get a large bucket (I use a 5 gallon bucket - but I tend to buy in bunches) (you can use whatever container holds your fabric) and place the fabric in there.
I only put one items of the same color in at a time in the soak. so in case there are dye issues, nothing is really ruined.
If I have colored water, then I take out each item - one at a time - and place in a separate container - to see which piece has the bad manners.
After that, I wash for a minute or so on a gentle cycle and machine dry at warm.
I try to avoid much agitation, especially the ones with the metallic (gold or silver) printing.
I washed some off when I was starting.
I have learned so much the backwards way. No need for everyone to do the same dumb things.