What to do about slander in the workplace?

After several years of great reviews at my work, I noticed that my supervisor suddenly stopped trusting me, and one of my co-workers was making comments which leads me to believe that someone has been slandering me and putting my job in jeopardy. I've tried to find out from my employer (they won't tell me), and I've even approached labout canada to find out if I can force them to tell me (which they said I cannot). I can only think of one person who may have held a grudge against me, but since this was someone I barely knew, and the reason was so insignificant (I had turned down a couple of jobs where I would be working under him), that it makes no sense. Other than that I have no idea. I'm afraid if I pursue this too much, I'll be fired for being a troublemaker and get a bad reference, and if I don't pursue this, I'll be slandered, then fired and get a bad reference. I'm not sure what to do. I've even considered looking for other employment, but I know some people at work will spread rumours around town that I left because I had something to hide, which isn't the case at all. I'm just tired of working for an employer who doesn't seem to trust me, and where I know someone is trying to ruin my reputation. What should I do. I just want to know where this is coming from so I can put a stop to it. Any suggestions?