I find that it always takes at least a week for the boys to reset their body clocks after being OS.
I can't advise you on the cot issue because we co-sleep but I'm sure that if you make changes gently she'll cope.
It's always hard to adjust after a holiday but I think for our littlies it's even harder because 3 weeks is such a long time in their time frame compared to ours. If you have a nap when she does during the day it might help you to catch up on your sleep - when I'm rested it's always easier for me to cope