I used to crash around 3pm, too. Always felt like I needed a power nap, and took one most days. I think in my case it was a combination of poor sleep from sleep apnea, hypothyroid, poor blood glucose control, and eating way too many carbs all day.
It took about 3 months of primal eating and 40 pounds loss to get over it. I now feel like a champ, sleep all night and never nod off in the evenings.
I remember feeling panic if I had to drive more than an hour, I'd need 2-3 Red Bulls an hour just to keep awake and I'd still catch myself 'dodging punches' while driving.
If you've only been at this a couple weeks, give it time. Have you had your bloodwork checked? Fasting Glucose OK?