How old?
I had that with a doe that normally has 7-11, but when she accidentially met the buck just hours after a litter she had only 2.
So, condition can be an issue, also overweight, mood, I assume maybe stress, bad sperm quality (heat...), and there's always pure chance meddling with our plans and expectations.
Since it does happen, I just would take it as an oddball litter, and see how she does next time, and not worry too much about some serious reason, like uterine cancer and such, I would deem that quite unlikely.