Most likely someone has pinned it on Pinterest or a similar site. People seem happy enough to do that even if it's an 'all rights reserved' photo - they seem to think that if it's on the Internet, it must be OK to use it. I've been in a similar situation where the same 3 or photos come up top but it's because I publish most of mine under a Creative Commons licence to allow blogs and websites to use them. It's possible that this has happened with your photo; if so, a Google image search might throw something up.