Female rabbits can become pregnant from as early as 10-12 weeks of age. If you haven't already separated the doe and the buck I would suggest doing so as she can become pregnant again already.
I'm not familiar with the care of kits, but putting them in a nest box with some hay to keep warm (and some of mum's fur from her belly - you should be able to tug gently and have fur pull out, if not, don't forcefully pull her fur). You'll need to make sure she's feeding the kit, they get fed once or twice a day, morning/evening. The belly will be well rounded and quite white in appearance. If the kit looks as if it hasn't fed, you may need to assist by holding the mother and then holding the kit to a nipple so that it can feed.