I work in HR so can provide some guidance (I think!).
Have u received any additional payments you don't usually receive per period/month (car allowances, bonus)?
Do you get paid per period (i.e 4 weekly) or per month?
If paid per period (4 weekly) has this year brought an extension to the number of tax weeks before the end of tax year 2011/2012 (I.e: does your most recent payslip read tax week 53, 54 or 56?)- which means you have been taxed in isolation of the latest 4 week period disregarding your tax paid in previous periods throughout the tax year, or in other words on a 1 week/1 month basis and not on a cumulative basis (in a nutshell your tax code may have changed for this 4 week period only leading to you paying more tax then normal as it is not on a cumulative basis).
Either way if you think you have paid too much tax, all you need to do is wait for your end of year P60 to be issued (legally your employer has to issue by 31st May), contact HMRC with this information and an adjustment will be made end of tax year.