Here are a few general guidelines (in my nonmedical opinion):
Does this interest interfere with your ability to function normally?
Do you lie awake at night thinking about this topic?
Does this topic cause major strain in your relationships with family and friends?
Do you find yourself doing things because of this interest that make you uncomfortable or that you wouldn't normally do?
Have others noticed or commented on your excessive interest in this subject?
Do you dream about this subject on a regular basis (meaning that your subconscious is engaged in thoughts that include this topic even while your conciousness is at rest)?
Do you want to decrease your fascination with this subject but find that you can't?
These are just a few questions to help you decide whether you need to mention this to someone. If you answer a strong "yes" to several of them, then talking to your doctor is probably a good idea. Obviously, only a doctor can help you truly decide whether you have a problem. These suggestions, however, may help you decide whether you need to see that doctor.
Remember, you can always start by mentioning this to your family doctor during a routine visit or an appointment for something else. Family doctors are used to fending unusual questions and can often help point you in the right direction or give you a useful referral. Good luck!!