I'm an incoming Freshman at USC.Yesterday I was biking to school like I had been doing for a week prior,along with several other people, when I and a couple other people, not all of them, was stopped by police. They gave me a ticket for biking on the wrong side of the road. They didn't give me warning. No signs had been posted around and USC didn't tell me either. Is this constitutional? Doesn't expostfacto prohibit them from giving out tickets without making it known that you can't do it

The situation you describe is constitutional.  Ex Post Facto is the creation of a law or raising the severity of the law and holding someone to that standard when they broke that law prior to its creation.  For example, if there were no bike laws when you rode on the wrong side of the street, then the next day the law went into effect, the police could not give you a ticket because what you did wasn't a crime when you did it. 

The bicycle laws, though, have been established prior to the events that you describe. Unfortunately in this situation, it is an individual's responsibility to know the law.  Ignorance of a law is not a defense.