Owe nearly £900 but just use PAYE

Havent changed jobs or anything. Just had an email stating I owe £900 there is no info as to why it's just say adjustment. Why doesnt it at least say why?

Hi Darren,
Most calculations should offer a brief explanation, but yours this may not have been included, unfortunately we are unable to check personal records via the forum, which this query would require, you would need to contact HMRC. Income Tax: general enquiries.
Thank you.