Tax Code and 2 incomes

I took early retirement in October & had tax code 1257. Two weeks after I retired my tax code dropped to 11657. Info says I have underpaid tax ? But also I have two incomes, which I haven't, just my pension. I tried the online chat but all they told me was what I can see online.

Hi,

An overlap between your last wage and first pension payment may have caused this underpayment, although there could be other explanations.

Since contacting our webchat service, we may now have more information on your record to explain the underpayment. We would suggest to contact us again by phone or webchat so we can check.

Income Tax: general enquiries

Thank you.