Could my 18 mth old rabbit be pregnant?

We have had 2 rabbits for 18 months and we believed they were both female. They often do the mating actions and now one is now nesting and has gone off her food. Is it possible that the other is a male and they have only just mated? or could it be a phantom pregnancy? I would have thought we would have had a few litters by now if one is male?