It's this:
http://www.nationalrail.co.uk/service_disruptions/currentAndFuture.aspx?TravelDate=28%2F10%2F2017&TOC=awElectrification works between Newport & Cardiff, 23-29 October

It also mentions (not very clearly)
BPW» -
NPT» closed each weekend, replacement buses which are extended to
CDF» when NPT-CDF is closed, so I think the latter is more the cause of your ticket anguish! Might be worth considering an alternative to the train that weekend!
I suspect it is also half term that week which may also have some impact on pricing.