Macbook Air 13" started downloading files by itself

I am a very time user for a MacBook its only 2 months old and I was doing my math homework online. I have been using the same websites since the day I got it and I was using this other website for problem solving in math and I been using it as well on my MacBook and today I was just on both websites at the same time. I started to see all these files being downloaded by itself they were called "unknown" and I got this black and white safari logo on my bottom screen and I couldn't do anything. it was saying I had a "backdoor virus" and it comes from bad websites and adult websites. I was very confused by it. I couldn't exit so I forced quit and it had asked me to send a message to apple and so I did but I don't know if my MacBook has a virus now? I just got it so I hope I don't have anything. keep in mind I was just on my math website and problem solver website that I use all the time. I hardly use my laptop it is just for like school and watching Netflix or Hulu. I just don't know if I should be worried that I have a virus?