No, the sherriff does not need to contact you when a report is filed involving you. They might contact you as part of the investiagation, but not necessarily. You can hire a public defender to represent you and force the issue to get them to charge you.
I would have to agree that the police are not required to contact you about a pending arrest. If they were, how many people would actually be around when the warrant was served?
I, like the other posters, would suggest that you get a lawyer. Obviously the police and district attorney has something, or the arrest warrant would not have been signed.
I agree that a lawyer is immediately needed in this case. As far as I know, the police aren't required to contact the accused before an arrest is made. They will usually investigate a claim, but I do not believe they have to inform the accused that there has been an accusation. Unfortunately, false allegations do happen. The police will do the best they can to sort out and investigate any accusations, but they have to do their job. The accused does have recourse that may include filing civil suit against the accuser as suggested above. Each state has it's own specific set of laws. It will be important to speak with a lawyer in your area that is familiar with these laws and can help you take the next step.
It sounds like you need to immediately contact an attorney to help you deal with this problem. It could be that someone convinced law enforcement officials that their charges were truthful, and then a warrant was sworn out for your arrest. You may be able to have these charges overturned, and a civil suit against your accuser could be a solution.