Have you changed jobs?
W1/M1 means the allowances do not take into account the previous earning in the financial year, so each payment is looked at in isolation.
Has the code changes as well?
Unusual to go from cumulative to non cumulative on the same coding.
My tax code has changed, it now has W1/M1 on the end, which Googling tells me is an emergency code - why would this have happened?