Not a stupid question at all! So when you are using the “Period” in the slicer, and you have it interacting with any of your tables, it will override any time intelligence calculation you have, unless you do something similar to what I did with your measure. I used the ALL function, which in the simplest of terms means ignore the slicer and give me the results of Sales LY regardless of what the slicer is telling it to do.
For more on the ALL function, go to this site within Enterprise DNA ( I would spend a bunch of time in here going over the different functions):
https://info.enterprisedna.co/dax-function-guide/all/
Thanks
Jarrett