Unravelling at the seams! Help!

I made a cute little nightgown with flannel trim (ruffles around the arm holes and bordering the bottom). I finished the seams inside but my friend said when she washed it the flannel part unravelled and came apart from the body of the dress. What did I do wrong?! Thanks in advance - you guys have so far proven to be very helpful!. 

Unfortunately, flannel unravels.  If you used the flannel as a trim, you should have finished its edges too.  Maybe next time, do a rolled eged on the flannel before attaching it to your work.  I hope this helps.  I'm sorry that it unraveled on you. 

Prewashing it helps - flannel shrinks a lot, so it is good to launder it a couple times before cutting. It tightens it all up so there is less ravelling -- but you can't eliminate it all, so as the previous poster said, finish the edges!

Well, I did prewash... but maybe doing a rolled seam and then sewing and finishing the seams again would help. Thank you! I shall try again.