I asked for the tax to date figure so that I could work out your new tax position on code 138L. However, the reason your code has changed is because you are receiving state pension. State Pension is paid gross but it is taxable income and HMRC have changed your code so that the tax on your state pension is collected via an increase in the tax deduction from your salary.
As you are receiving £177.31 per week, the tax deduction from your salary should increase by £35.46 per week (£177.31 @ 20%).
Your tax code has changed from 1060L (£10,600) to 138L (£1,380 to £1,389). That's a reduction of £9,220 to £9,229 in your allowances. £9,220 is what you would receive in state pension in a full tax year but as you were 65 on 30 May, you won't receive that much so you should call the tax office on the number here
and have your tax code reviewed and changed while you are on the phone to them. I know the wait times are too long but phoning is the quickest way to have the code changed. Try calling at 8am as soon as the tax office call centres open.
I hope this helps but let me know if you have any further questions.