Unexpected K5 Tax Code Change In December

Hi, I was contacted by HMRC about underpaid tax from a previous year in December and moved over to a K5 tax code for January to March this year paying extra tax and was advised that my annual tax allowance was for last tax year had been reduced from £12570 to £-65. When I complete self assessment for property income this means I'll be taxed also for the negative annual tax allowance but have no idea why this is as my circumstances have not changed.....is this perhaps due to fraud. Please help.

Hi DAVID SMITH,
To advise you correctly, we may need to review your code and return, and discuss your query in more detail.
To allow us to do this, contact us by webchat or phone here:
Self Assessment: general enquiries
Thank you.