I only have one job why am I getting taxed for two?

I only have one job sometimes I do overtime for them. But this is not a new job why has my tax rate been cut from the normal 1271L? Why does the HMRC think I have a second job which pays more than my only job? Help?

Hi Vrhodes,
You will need to contact HMRC to review and update your tax code.
Details of how to contact HMRC at Income Tax: general enquiries
Thank you.