I've been feeling more tired and have some flashes of light. When should I see the doctor?

Soon. Feeling tired and seeing flashes may or may not be related. Anytime you see new flashes, you should be seen by an ophthalmologist within a couple of days.
As soon as possible. While tiredness is usually not associated with flashes of light, it can be associated with lightheadedness (which can sometimes give the appearance that you are looking through a glass of soda). Flashes of light tend to imply a retinal issue, and although the condition is benign the vasst majority of the time, there is a risk that something more serious is happening. See an eye doc immediately.