It could be that you haven't earned enough yet to reach the tax threshold, or at least it hasn't been recognised in the system yet that you will be earning over the minimum threshold, so they have taxed you accordingly. What tax code is on your pay slip for this month?
Same thing happened to me when I started my new job last October... I hadn't yet received my P45 from my previous UK job (I'd left that job 2 weeks earlier), so I was taxed only about £40 as if I had not yet earned any money in the tax year. A few days later I did receive my P45 and gave it to HR... and the next month I they took nearly £400 in tax and NI, to make up for not taking enough the month before!
It could be that it will be rectified in your next pay cheque, or it might be worth talking to HR at your company to see if there's anything you or they need to do to make sure you're on the right tax code and are paying the correct amount of tax.