I am not sure how I can show an image here since there is no option. Mastoid is the bone just behind your ear and if you trace downwards, you can feel the tip of mastoid with your fingers just before the soft tissue of the neck begins. Mastoid, like many bones forming our skull, contains air cells. These cells are kept moistened by normal secretions. Sometimes fluid can be collected in this cells if there some obstruction to this secretions, or inflammation or infection. A small amount of fluid without any redness, pain or fever usually does not mean infection. This may be a variation to normal secretion. However, judging from you age and symptom, I would have really sought for a contrast injection which could not done in your case. Please consult the referring doctor for evaluation of any infection or any chance of obstruction.