Personal allowance reduced by £5,072 - why?!

I don't understand why my Personal Allowance has massively reduced from £12570 to £7,498. Apparently my tax free amount has dropped by £5,197 (due to underpaying £ 1,790 in a previous tax year). Help - this seems so grossly wrong!? Last year I was owed money back and now suddenly I am the one owing. I don't understand (a) I would like to understand why and how I apparently have underpaid tax, and (b) why have I incurred an additional £5,197 deductions from my personal allowance - there seems to be no explanation why. I'm an NHS worker and I really can't afford to be taxed more than I already am!!!!!!

Hi charlottehowland,
To give you the correct advice regarding your underpayment, we would need to review your record.
To allow us to do this, please contact us by webchat or phone  here:
Income Tax: general enquiries
Thank you.