As everyone is saying, it's called paranoia. It can also be a symptom of anxiety/GAD (Generalized Anxiety Disorder), or a combination of those conditions. If this is how you feel all the time, you REALLY need to get a psychiatric evaluation, diagnosis & treatment. There are many medications and/or different types of psychotherapy that will help you a LOT. Make an app't. with a psychiatrist or psychologist ASAP! The sooner you get help, the better your prognosis will be... Best wishes! :)
I have been paranoid but have not felt like someone was watching me. It could be a side effect of a medication, such as prednisone, or a drug, like marijuana. If this persists, please seek the guidance of a professional and don't suffer in silence.