Someone using My email address to spam

I received an email from MY email address that my spam filter put in my spam box, but I didn't send it. How do people send emails like this, and how do I prevent it?

I use Yahoo/AT&T for my email. I checked my "sent" folder and that email was not in there. I also right-clicked the address to see if there was a "real" sender in the code, but it was my email address. I used to be able to mouse over a return address and see the real sender. Wish that feature still worked!!!

Just as a precaution, I reset my password, although I really don't think I've been hacked - just impersonated. :(

Thanks so much for any suggestions!

Peachy