Teething is one of them. Sometimes it is very obvious (tooth cutting through, red and sensitive gums) but it can also be more hidden. Check this full list with signs of teething to see if this may be happening.
Another good possibility is separation anxiety. It's a typical thing that can pop up right now. You may not notice too much during the day, especially if you spend most time together or if she is with a carer she is very much used to. But nights can be different, dark and lonely.
It could also be as simple as a little growth spurt, something special going on in her life right now (going to day care, a move, an exciting or stressful time in the family, ...), a change in the weather (too hot or too cold in her room), ...
Let me know if you recognise any of the above - just post a comment below. It will also help if you could give a bit more details: does she cry when she wakes up at night, does she settle by herself again or with your help, does she want to drink or feed, ... ?
Has anything changed in her rhythm during the day? Does she take less or more naps? Anything you can think of that changed lately?
With a bit more info I'll be better placed to help you track down where the poorer nights came from and what we can do about it.