If you don't want babies, bunnies shouldn't be introduced if they aren't neutered and spayed as they are really really fast!
Like someone mentioned, if she was with other bunnies before, she could have gotten pregnant then. However, if you haven't seen her being mounted, it's possible that she is having a pseudo-pregnancy as well. That means, she would think and behave as though she is, without actually being preggers.
However, since she has been around other rabbits, I would consider it a pregnancy until proven otherwise, just in case. I would keep an eye on her food intake, water, and provide her with lots of hay and just let her be to do her thing. I would keep her away from any other bunnies and count the last time she had access to a buck as day 1. If she passes gestation period and you haven't felt any babies or she hasn't given birth, then you can decide what you want to do.
Since we don't know when she would have gotten preggers,I would put in a box so she can nest. I would also try gently palpating to see if you can feel anything. SHe might get a bit aggressive so be gentle andavoid startling her if possible.I am sure there are articles in the Library section if you need more info too.
(Some people might suggest you have her spayed now if it is an unwanted pregnancy and this would result in the babies being aborted ifshe would indeed be pregnant.That is a controversial topic so I will let you be the judge of that.)