My Personal allowance lowered

I have been in the same role at my job for over a year now, however my personal allowance is lowered to £9,800 instead of the usual £12750 why is this?

 Hi 
Unfortunately, it is not possible to check individual tax records through this forum, if you have a question which relates to your personal tax situation or tax code please contact our helpline:
Income Tax: general enquiries
On this page you will also see a link to: 
Personal tax account: sign in or set up 
which may be the quickest way to review information relating to pay and tax history, tax codes and how to change them.
Thank you.