Ah, you mention tea... Sweetened or unsweetened? How many grams of sugar are you getting a day?
Many people don't seem to realize this, but all carbs are not created equal. Everyone is different, but whole grains are generally fine, and can be an excellent source of fiber. It should not be a main source, but a whole grain bread, low in cal, used to make a sandwhich is healthy for most people. White bread and sugar are not. When I cut out my rice krispie treat (100cal) from lunch, I stopped feeling like napping in the afternoon.
Extreme tiredness can be a sign of many things but the most obvious would be that you're either not getting enough food (energy), or suffering from either a caffeine or sugar (or carb, since processed carbs = sugar) crash. It may also mean you just started running marathons, but I assume you would have filled us in on that.
Since there are medical reasons as well, I would wait another week, maybe make a change or two, but if it doesn't get any better, see a doctor. Just to be safe.