So I have now used Memtest. I had completely forgotten that Memtest just continues unless you cancel the program yourself; it ran 5 hours 
. I also forgot to save the result somehow - I installed the Fedora package but I think I still have to save the results separately somehow. Anyway, there was an error. This is bad. I’m going to run Memtest again tomorrow. After that I’m going to post a screenshot/photo here. Cya!