TroTro

Hi Hope you can help. Recently changed jobs, I got pay for one week from my old job and 2 weeks for the new one. Both were tax code 1257L. I then got a notification from HMRC saying I'd not paid enough tax in the current tax year(this year) and my tax code was changing until next year to 1112LX to pay it back. Im confused how i owe tax now as id been paying correct amount. Thanks for any assistance!