Phones storage

So I have a iPhone 12 pro and it only has 128GB but I love taking photos and playing games on it so I always take up a lot of space on it. I had just gotten a lightning cable USB drive to take off all my photos and save up some space. Now at first it was working great but my storage kept going down from 70GB to 45GB and then to 13GB. I was a bit worried but didn't think it was anything terribly bad cause the storage bars weren't down that much. Then a day ago I had seen the storage shoot to 18 quadrillion GB! I'm still able to download apps and take photos but it won't download the new software updates and I'm just wondering if anyone knows how to fix this.