Sarcasm not necessary. Paranoia would be thinking you're being followed when you're not, but I think you know that.
If someone, who is a stranger to you, is following you---it would be reasonable to fear for your life and/or safety.
I wasn't being sarcastic. A unwarranted fear of others is paranoia. There is no reason to fear for your life if someone is following you. I dare repeat myself in saying that if being followed is good grounds to "fear for your life" then the next time I'm at the mall and someone follows me around the corner I'll just pull out my gun and start shooting.
par·a·noi·a
noun /ˌparəˈnoiə/ 
A mental condition characterized by delusions of persecution, unwarranted jealousy, or exaggerated self-importance, typically elaborated into an organized system. It may be an aspect of chronic personality disorder, of drug abuse, or of a serious condition such as schizophrenia in which the person loses touch with reality
Suspicion and mistrust of people or their actions without evidence or justification