Quite often, nothing much until there are babies. Especially if mum is largish & plumpish.
Just before the birth the mum makes a very cosy nest with her own fur, & the bunnies live hidden in and under this for a week or so. Don't disturb her or them though, as this can trigger her to kill or even eat them. Wait till their eyes are open.
Has she been with a male? If so, don't worry, she's pregnant...
My rabbits have been mating but when we bought them I was assured that they were both female. Now there has been a nest built of fur and one rabbit is considerably bigger than the other! What shall I do and might 1 be male and the other pregnant? Thanks