I have hens that do that same grass or straw thing when they are just about to lay an egg.
Your bird may be broody -- a few days will tell you more. I've just had a hen come out of being broody, so this is fresh on my mind. Here are some signs of a broody:
If she runs back to the nest and just wants to sit there continuously - with her wings slightly extended -- she's broody. Other clues:
1) a bare red patch of skin on her chest/belly area where she's pulled out her feathers, so she can keep the eggs really warm against her skin
2) being more aggressive with the rest of the flock
3) her body temperature seems very high when you pick her up - like she's running a fever
4) big, humongus, terrible horrible-smelling poops, when she is off the nest (they make you wanna puke, they smell so bad)
5) her comb may get slightly less red, like more orangey colored
6) she screams & growls at you or the rest of the flock, when you get close to the nest
7) when moved off the nest, she crouches, jiggles around & fluffs her wings ( wherever she is put), as if she thinks she is still on a nest
8) she no longer squats, like hens do for a rooster
9) she, eventually, stops laying eggs
