It's possible she's having a "false pregnancy". Had that happen to one of my breeding girls once....she had not been bred, but nipples pinked up, belly swelled and then after a month belly went down. Another possibility is she has resorbed the fetus. It's also possible that your boy is not neutered, or maybe not done correctly and completely? Pregnancy lasts about 63-68 days, so you'll know for sure by then. Give us an update whether your girl has kittens or not.
Wildest, craziest, most out-there guess - parthenogenesis! No, probably not. False pregnancy is far more likely. But if she does give birth, the kittens are all female, and they all look like her, you've got parthenogenesis.