Please show how you setup the relationship between the tables.
As well as the relationship in the queries

Do you have any code/macros doing anything in the background? Or can you provide more pictures? The data wouldn't just change without some kind of intervention, but that piece of the puzzle is still missing.

Is that form set up so that you can edit records in it and you've got it based on two tables? Because I'm guessing that you're editing the category on that form and you may be thinking that you're changing in what category the product is in, but you're actually changing the category table. This is why I always say that your forms should be based on only one table, and any other fields from other tables should be locked for display only. Too Many Tables

here is an example of what is happening. The catagery ID changes on a product so it then shows up in a different category. In this case from category 1 to category 20

To help you understand Richard's reply, what is the form bound to? The product table or the query?

The form was also set to the ProductQ so I have now changed that back to taking data from the ProductT.

If it is bound to the query, that is your problem. Bind it to the product table.
And have the product category in a combo box.

That's exactly what I now have. I originally had the ProductF bound to the ProductT but bound it to the ProductQ as I wanted to also include in the ProductF the "mark up%" and the calculated "Our Price" which is calculated in the ProductQ. I have now removed the fields for that in the ProductF but left it in the ProductQ, as may be I can set up a seperate form just showing that?

I learning slowly. The tech help videos taught me how to set up this database that  I use now in my workplace/business. I have studied for hours and hours and adapted some of the things to get things to do what i want my database to do. It has been one of the best tools in my business. I would like to take this opotunity to say a BIG THANK YOU to Richard for such a great tool and way of learning. Im now hooked and hope to create an even betet database using some of the more complex tools/coding functions etc.