K.Y.
I think it's possible that she is going through the 18-month regression in sleep patterns. My daughter went thru a terrible phase at 18 months and when I started asking around, people said it must be that "18-month regression thing." I also think teething may play a role if she happens to be getting her 2-year molars early (or other teeth if she doesn't have them all in yet). Are you down to 1 nap per day? Probably time, if you aren't already there.
--K.