I've had people tell me I was faking it before. One was some know-it-all at a family get together. I basically threatened to punch the jerk-I don't recommend this approach for obvious reasons! Besides that one, at one point my father started in on me with the whole faking it thing. I told him if he wanted to no longer have a son, to just keep it up. I threatened to shut him out of my life. I would no longer have any contact with him-ever. Fortunately, his reasons for saying this were rooted in denial, and a fear of his only son being sick, rather than an attempt to just be mean. He got past that, and now he's one the most supportive people in my life.
As far as your situation, you are being harassed on the job. There are laws against this, and your company should also have policies prohibiting harassment as well. I would go to management and tell them you want this person stopped immediately. This person is calling you a liar, and nobody should have to put up with that in any situation. I would also document the harassment, in case management won't help you, you might want to take some sort of legal action.
And if none of this helps, just let me know and I'll have my cousin Guido from Jersey send her a dead fish! (insert sarcastic smiley face)
PS-I know from your other post that you are having a tough time right know. Please know that you can come here and vent, or just talk, anytime you like. There are literally hundreds of people here who have been in your shoes, and understand what you are going through.