If i have a fear of something but don't know what it is exactly, does that mean i might have an anxiety disorder?

Yes: Yes fear could be one of the underlying symptoms of anxiety. I suggest meditating for 10-15 mins (daily deep breathing and relaxation as well as clearing your mind) to see if you can discover the reason behind your fear.

Possible: Fear that has no object is certainly a sign of anxiety, however it also can be a sign of other mental disorders, in order to get a clearer answer , you may want to get evaluated by a psychiatrist to rule out other causes. After that they can direct you to the best course of action to control the symptoms feel better.