low memory virus

Please review this recent Discussion, because there are scams that cause a dialog box to appear alleging your Mac is low on memory when it isn't:

MacBook Air low on memory?

If that dialog box resembles the one in that Discussion then it's a scam. Don't act upon it. More information will be required to determine what's causing it, and how to eliminate the scam.

Please review that Discussion and write back.

It's very easy to determine if your Mac's performance is being limited by available memory. Please read Use Activity Monitor on your Mac - Apple Support. If the "Memory Pressure" graph is frequently "red" your Mac will benefit from additional memory. If not, it won't. It's that simple.

This is unlikely to be caused by a virus, as the subject of your post would suggest.

I will speculate that immediately after the Catalina upgrade your Mac was doing a lot of self-maintenance, as it were, in the background.

While the "low memory" warning disappeared when you cleared your Safari cache and history, I think that's more likely a coincidence. That is, I don't think the two are related.

Before worrying about it further or trying any other fixes, my advice would be to keep the Mac running for a few hours and see if the problem goes away by itself.

As a general rule, it's always advisable to upgrade your operating system because it's one of the best ways to keep your Mac secure. So I wouldn't say it was a mistake, even if there are teething troubles.