Gmail issue

I have a gmail account that I mainly use for junk mail. But it does come in handy. Every once in awhile I notice that emails come in and somehow there is a response going back to that email that I didn't send.

It doesn't seem to do it all the time. I have changed passwords in the past and just did it again, but it doesn't seem malicious, rather almost like it's an auto-reply thing I have set somewhere or something.

Any ideas? It doesn't seem to be any kind of virus or something that is sending random emails to all my contacts either, just replying to the emails sent to me which, like I said are mostly junk anyway.