Okayyy, then you really shouldn't be getting such messages. They'll appear when you get down to your last couple of GB, not before.
But just for clarity, I asked how much you had free space rather than how much you are using because (a) it's the free space that matters here, not what's used, and (b) usage is ambiguous: some people will say what they personally are using but not what the system is using (which they may not even know, or even realise is a thing), and this can cause confusion.
But if you have 10s of GB free then fluctuations in caches won't bring you to the point of triggering the low storage message. Which makes me wonder whether this was a real system message or an advert pretending to be one in order to push you to install some junk app? Can you describe exactly what the message looked like, what it said and in what context it appeared? Sorry for all the questions, but as I say it sounds like there is no way the system storage warning should be appearing, which is why I'm wondering whether it's possible it was something else.