relationship advice

Just looking for advice or opinions on an issue with my GF that I'm completely perplexed about. I'll try to keep it brief. I came home from work last week and I noticed there was a females ring sitting on my banister. I assumed it was my girlfriends ring as no other girl has been to my place in months. Well when my GF got there I grabbed the ring and said hey you left your ring here. She said it's not her ring...and that's when all hell broke loose. Now she's asking me where this ring came from and I don't have the slightest clue. I don't know what to tell her other than I don't know. She's certain that I had another girl over, which couldn't be any further from the truth. I've never cheated on any girlfriend, and I definitely wouldn't cheat on this one as I love and care about her more than the others. I've entertained a few ideas; maybe my last ex somehow got in and is trying to sabotage my relationship, but I don't see how that's possible because she doesn't have a key to my place and I've haven't heard from her in a long time. I've also considered the possibility of my current girlfriend planting it there as a reason to get out of the relationship, but I don't think that's likely because she's hasn't left me over the issue. So now my girlfriend is asking me over and over where this damn ring came from. I keep on telling her over and over I don't know, because I don't...I don't know what else I can do...