Hi there! I know exactly what email you're talking about. I've gotten several of them recently, myself. You are not necessarily infected, but it is possible.
First off, you state that your computer was "unprotected" at the time. However, does your email client have any type of spam-control software on it? What email client are you using? If you have a "spam" folder, or anything of the sort, check there and see if the email is there.
If you are concerned about the possibility of being infected, I recommend that you download, install, update and run a full scan with the free software MalwareBytes AntiMalware. This is an excellent program, and I know the creator personally. I run it on my machine at all times!
I'll keep an eye on this post, and hopefully we can get all of this figured out.
