HMRC 'in year tax adjustment'

Good Evening. Received a letter last week informing me that I owe HMRC over 200£ in unpaid tax due to a 'in year tax adjustment' I retired [early] from my work at the end of July this year and received my 2 works pensions on the 1st and 21st of August. My work had always paid my tax and as far as I am aware they are up to date with this. My combined pension income amounts to just under 12000£ per year so below the 12570£ threshold. However HMRC calculates my pensions to total over 20000£. I spoke to an agent on the telephone and she tried to explain but I'm still not sure why I have been penalised and sent this letter. I have anxiety issues and this has really knocked my confidence and would appreciate any help and advice re. this issue. Many thanks for taking the time to read this post

Hi dessy10,
We would need to access your tax records to find the reason an 'in year adjustment' has arisen. You can view and update your estimated income from your pensions online on the personal tax account, Personal tax account: sign in or set up If you still feel the adjustment is incorrect after updating your estimated income please contact the helpline Contact HMRC.
Thank you.