Hello, I am Keith, one of the experts on Just Answer, and happy to help you with your question.
Right, first things first; a suffix Y is for a person born before 6 Apr 1938 with personal allowance of £10,660. If you do not come into this category then there is something amiss.
I cannot tell you why your coding has altered, Just Answer is an independent tax advice site and has no connection with HMRC. I can, however hazard a guess. If you are in receipt of an occupational and a State Pension (SP) then the code is applied to the former as the latter, although taxable, is paid gross. Thus a reduced code number is ***** to collect the tax due on the SP. Alternatively you may be in receipt, or HMRC think you are in receipt, of income not taxed at source. In any event the Notice of Coding should give you some information as to why the normal 1066 is not being applied. If the coding is incorrect you should take it up with the HMRC office which issued it in writing and request that it be changed.
You should always check your code number on each and every time it changes as the correct number should make you neutral at the tax year end; at least that is the theory and it is surprisingly successful.
I do hope that I have helped shed some light on your problem.