A Dr visit wouldn't hurt.
When you said you've cut out fatty foods, did you also cut out carbs? That would have this effect as well.
Could be withdrawl, could be your body healing, could be your body was used to burning the carbs and sugers and whatnots of your drink of choice. Or maybe you're just catching a cold...
If it doesn't improve, go see a doctor, they'll most likely be able to pinpoint what it is, whether its related or not.
I waivered between lethargic and energetic in my first couple weeks...