OK, I won't state absolutely and unequivocally that your doe isn't pregnant. Rabbits have some pretty amazing tricks in their reproductive repertoire that makes it possible, though unlikely. However!
While rabbits don't do a classic "heat cycle' like other animals do, they do experience hormonal peaks and valleys. A doe at her hormonal peak may pull fur (usually, this will be a doe that is entered in a show in the next week or two - effectively ruining her coat for the show, lol). Your doe could just be signalling that she is ready to breed again.
Rabbits are what a known as "induced ovulators." A rabbit doe ripens eggs, but doesn't release them right away. They are held in readiness in the ovary, until a hormone released during the act of mating triggers the release of the eggs. A rabbit that gets mounted by another rabbit, whether a buck or even another doe, can experience a "false pregnancy." The doe that is experiencing a false pregnancy will refuse service from a buck, may be grumpy, etc, just as an honest-to-goodness pregnant doe would. Usually, the false pregnancy ends somewhere around day 21, but I have had a few does that went to day 31, made a nest, pulled fur, the whole bit, and produced nothing. Your doe could be experiencing a false pregnancy.
If the babies are 8 weeks old, and your doe actually is pregnant, that would mean that the kit sired a litter when he was only 4 weeks old. Sorry, not believing that for a minute. I do know that rabbits can breed through cage wire (which is why, when I have to house males and females side by side, I make sure they are the same breed) Rabbits can also do a trick called "delayed implantation," where the embryos don't implant right away. It might be possible for your doe to be pulling that trick, from a breeding with the buck from before you began housing them separately.