3 year old, early riser.

Amanda
For the last week or so my 3 year old religiously has been getting up at 6 am on the dot. When I try to put her back to sleep she throws a fit! Then she refuses to nap in the afternoon. I'm 4 months pregnant and could really use the extra sleep. What can I do to curb this behavior? And why is she getting up so early? It's still dark at that time where we live.