Hi M.,
You could be dealing with one or more of several different things.
1. The first, and most obvious, being what you mentioned--she may just grow out of it. However, that doesn't give much comfort now.
2. She could be coming down with a cold or other virus and you are just seeing the cranky beginning.
3. She could be teething (if she doesn't have all of her molars--that can always be fun!). Sometimes kids hold off on (or can control) their poor behavior when in the care of others because they know what is expected of them and then they really let their parents have the show. Or she is simply distracted enough during that time, and when it is relax and be with family time, she feels comfortable enough to say, Oh yeah, my mouth hurts.
4. Something could be happening at daycare that is disturbing to her.
5. She certainly could be so hungry that by the time she sits down to eat she doesn't know what she wants and rejects food anyway.
But like I said at the beginning, it might not be anything, and you may just have to wait it out. Have you noticed any other changes? Is her diet the same? Activities the same? Really, think about her entire day/life to think of any possibility of change that could be throwing her for a loop--even the really small stuff.
Good luck!