Where's the money?

Oh, you guys! I feel so butt-hurt LOL. This is the first time this has ever happened to me at a job. So at closing time, my boss and I are counting our drawers, and mine decides to come up  as 81 dollars OVER. Not short, but over. Having your drawer be either is still bad either way. Anyways, we realize that this is completely wrong and start scrambling our brains to figure out exactly where it went wrong. We ended up having to count all of the cash drawers that were used today and re-count our drawers, count the safe, ... all kinds of things. Well we didn't exactly calculate where it went wrong, even after calling our main boss lady. And well frankly, this sucks. Like I said before, this is the first time this had ever happened to me, and I know I didn't take it, but that it was left in my drawer. I just really want to know what happened, and the only thing that really sticks out to me is that I had a conversation with one lady customer for about 10 minutes. I know that she didn't really buy a lot but enough to get a big bag, and that she paid in cash. And that's where I'm thinking I might have not handed her back her change. But I COULD HAVE SWORN THAT I DID!! Ya' see? And even if I didn't, you'd think that she would have said something, unless she was so into talking to me that she didn't notice. Anywho, I didn't mean to do it and I feel horrible for having something like this happen, especially if it were my fault. It makes me not only feel bad but look bad too, because people can get fired for having their drawers over or under a certain amount. And 81 dollars is quite a lot to be missing/over. Oh well, I will still take full responsibility for what happened, because I should have been paying more attention either way.