I have often wondered that myself.
She's always been like that. She did that in My So Called Life too.
Her character in Homeland may need that expression used more often, so it looks more accentuated.
If you've ever seen her in Romeo and Juliet you'd know the answer is yes.
She quite young in it but I think that's where the pain expression originated. Think she thought it was a tragic shakespearean expression and it stuck