As babies mature into toddlers, then into school children, everything they do, want, learn, dislike is in a constant and steady state of flux - progress, maybe; sometimes maybe not.
As mental processes develop she will 'think' more about the day's events, and the day to come tomorrow. She may be more aware of sounds in the house, or outside, and think about those.
If she is content, and not worried at being on her own, or afraid of the dark, then consider yourself fortunate, as not all toddlers are so compliant!
Give her dolls, teddies, even a cloth book, in bed to comfort and entertain her; maybe phase out mobile if you conveniently can, so that toys are IN bed with her.
There aren't really 'rules' on these things, and although parents may have read the 'parenting manual' the babies haven't!