From time to time your chicken may go broody. In some chickens this maternal instinct is stronger than others and it can happen at any time. It is quite easy to spot because the broody hen will simply sit in the nesting box (or flower pot!) and refuse to budge. She may also make a peculiar growling noise if disturbed and become quite aggressive. However, unless your chicken has been near a cockerel within the last 7 days the eggs will not be fertilised and will never hatch into chicks. If you are not removing the eggs everyday there is more chance that a chicken will go broody.
What you should do
If you do nothing, your chicken will stay like this for up to 3 weeks (the incubation time for eggs). It is not necessarily a problem but it will prevent your other chicken from laying in the nesting box. You can remove her from the nest and block the nesting box so that she cannot get in. Be brave, open up the egg port and lift her off. After a couple of days she will lose the urge to sit on the eggs and you can open up the box again.