The presumption is the previous terms continue, but if they do alter them, they must tell you. However, if they don’t, and you don’t challenge it and keep on turning up for work as usual, you could be viewed as effectively accepting the changes. Which would cause problems if you try going to a Tribunal later.
As others have mentioned above, you need to speak to your manager about this in the first instance to find out what is actually going on here (i.e. whether it’s a genuine mistake, or they have changed your terms, etc), awkward as I realise that is. Otherwise, all you can do is speculate, which won’t solve your problem.