matleave,
were you working full time, all year, until you went on maternity leave.
The Revenue tend to put people on maternity benefit on a "week one" basis of tax - this means that they get their weekly/monthly tax credits and standard rate band, but they don't get the cumulative benefit of those - so if, for some reason you'd extra credits/standard rate band to use, then you lose that. If that is the case, requesting a P21 after the year end will sort it out (though you'd continue to have a cashflow disadvantage now).
If this isn't the case, it sounds like something has gone quite wrong on your tax certificate. Ring your tax office (with your PPS number handy) and talk it through with someone there.