It doesn't matter if you feel like you can diagnose yourself or not. If you're feeling weird and you don't like it, it's a good idea to see a therapist. An ounce of prevention is worth a pound of cure.
Even if it's just a dip in motivation or some low-level anxiety or something, it's worth it to work on it with a professional. Everyone has things they can work on.
One viewpoint is that "depression" often presents with only a small set of symptoms. For instance, in middle-aged men it frequently presents as only physical symptoms. Also it is very possible to have some of those symptoms and not realize it or have your doc realize it.