I plugged in my flash drive last night and all my files automatically loaded onto my desktop; I also got a popup saying my comp could not log into iCloud.

I bought a mac mini about 4 months ago, running El Capitan, then moved to a place without an ISP. This means I've been using a flash drive to carry work with me to school or the library, which has PCs. Last night, after using the thumb drive at school, I plugged it in and all of the files on it loaded directly onto my desktop. I also got a popup saying the computer could not access iCloud and prompting me for my password. Obviously, I have not been using iCloud since the computer is not connected to the Internet. Today when I booted ip, I was prompted again to log in to iCloud. I could not find how to disable this prompt from appearing on startup; I just had to select "later" to make it go away. I haven't made any changes that I know of to my preferences. Have I picked up a virus on my thumb drive?