Your baby has become a 'toddler.' Around this age, her brain starts hitting some major growth spurts - they will last until she's about 3 (hense the terrible 2's. It does affect her sleeping. And, as she begins to experiment with independence, she may resist sleep more.
So what to do ... I agree with other posts that she may be getting overtired, and that she might be coming down with a cold, and that she might be teething. So try to avoid/deal with those sleep issues. Also develop a routine, that you follow firmly (if she's really sick, there will need to be some adjustments, but otherwise, stick to it). It should be short (15 min or less) and can include stories, rocking, time in mom's bed (with the lights off!) or anything else that has helped her in the past. Just be patient and tell her "It's time to sleep now." She may wiggle (you can't really stop that) but she must stay lying down (just calmly and gently lie her back down if she gets up and tell her "its time to go to sleep") She may not 'get it' at first, but as long as you are patient and calm, and don't let her control you at bedtime, you'll be setting up a good standard so that when those 2's start and she really tries to assert her independence, it won't be a new thing. And in time, she will grow out of it and develop sleep habits and routines that don't need you right there.
Boy, don't kids grow up fast! :)