Receiving unexplained emails from own account

For the second time in around a month, I've received an email which says it's from my own account (it shows up just as something I've knowingly sent myself does). The title is '8/26/2013 4:35:38 AM Document' (the previous similar email just had a different date/time); I've never sent myself an email with that subject. I've not opened this one as I'm concerned about where it's come from, and am determined to find out what it is before I open it. I opened the previous one (somewhat absent-mindedly) - it was a blank message with an attachment, which I didn't download or open, but I could see from the little preview image it's nothing like any documents of mine. I've not experienced any other hacking-like issues, no-one else has received unexplained emails from my account, this is the only unusual thing that's happened, but I want to know what's going on largely for peace of mind. Any explanations welcome!