What is happening if I'm getting junk mail from my own email address. Is someone using my address?

Probably not. What some spammers are able to do it 'snatch' your email address and 'stuff' it into their 'sender' slot as well as the 'recipient' slot on the email, so it looks like it is coming from you as well as going to you. This is just one of many tricks spammers try to get past the spam blockers. Just delete the email.