peeples....I am so impressed with your continual awareness and concern for your children's every need. This is a blaring sign of an excellent Mom. It's been years since I've had to use Mommy skills where toddlers are concerned, but kids don't change much at all, or so I'm told...LOL!
At 21 months old, this issue can be due to about a hundred different things!......Can I just say, by the time you figure out what it might be and come up with a solution, she'll be over it entirely....and on to the next "new" activity!! Bless your heart. I'll just throw in a suggestion, for whatever it's worth.....When your little Angel wakes the first time, after you've done the "potty thing," made sure she's not ill and that she is warm and comfy......Place a chair next to her bed for yourself.....lay her down and stroke her head or rub her back as you sing or talk very softly to her....mostly reminding her it's time to sleep.....when she seems just about to doze off, give her a kiss and quietly exit. Each subsequent night, shorten the length of time you stay, always stating "it's time to sleep."...until the time that you simply pick her up and take her back to her bed....and cover her up, reminding her, "it's time to sleep."
I did this with sons #2 and #3.....and it worked like a charm. If you want to know why it works........I have no clue. The usual "hit or miss," parenting style!!!