Walking back to my computer (Windows 10), I noticed that my browser's history tab had just opened without me touching it and I thought I saw my cursor moving around. I had no control over my mouse until I ctrl-alt-deleted.
When I came back I had multiple new tabs open to things like ps4 digital game cards bought under my account and confirmation emails saying I just had bought a few hundred dollars worth of them. I also had a confirmation email about an account created with my email on "gameflip.com". I've never even heard of this website but it looks like its a place where you can sell stuff. My credit card company sent me an email detecting fraudulent charges and I dealt with that so I think I'm good on that end. Also, changed my passwords and all that jazz.
The problem is that I can't figure out how they did this. I've scanned my computer with Windows Defender, Malwarebytes, and Malwarebytes Anti-Rootkit. I also ran Malwarebytes in safe mode as well but they all found absolutely nothing.
Someone suggested I go to grc.com and run a scan to see if I had any open ports, but they were all in stealth mode except 21 which was in closed status. I'm not super tech savvy so I'm not sure if that means anything but as far as I can tell, "closed" shouldn't really be bad right?
If someone had just figured out my credit card number and passwords then whatever, that's easily solved, but how do I stop them from just taking control of my computer again?