Sounds like a vitamin deficiency or too few carbs for your activity level perhaps. What do your typical meals look like?

Consider that you are making a successful metabolic transition from burning sugar to burning fat. This amounts to perturbing your body-mind's homeostasis. Just as it can be uncomfortable to start a new exercise program: 36 hours after a longer run than usual, you might find yourself feeling "lousy." Until your body begins to say: "Hey, that run the other day - how about we keep that in the ongoing plan?"

All the same, the most precise homeostasis is rigor mortis. So even when making "paleo" choices, don't settle on complete uniformity. Keep tweaking things.

I (and most low-carbs paleo newbie) experienced the same, it's called "The Low Carb Flu", it may last 1 week.

Read about it at MDA

Warning: Don't give up and binge on carbs, stay motivated, eat some (I said some) berries for snacks and it will pass and you should feel as your 1st 3 weeks very soon.

Keep up !!!!

I had this problem recently and I'm no expert on this because I have just started myself but here are some things you might want to look into:

-Vitamin Supplement - Take one of any brand a day until you read up and find the right one for you.

-Fish Oil - Once I started to counter my Omega 6's with getting more Omega 3 from diet and fish oil I felt better, again just start taking them and then you can adjust and find a brand you prefer.

If your going to have a cheat meal make sure its gluten free - I did this on week 5 of paleo after feeling great I had taco bell and some bread with breakfast. I didn't think it would throw me off but I guess it did. Apparently it can take almost 2 weeks for the body to heal itself from gluten after its been reintroduced. Not sure about the exact science but thats what I read. I DO know that it felt like starting week 1 (carb flu, foggy, low energy) again. So I suspect feeling good has a lot more to do with a healthy gut than I thought.

-Enough Protien/Good Carbs/Calories Are you eating enough? I found I wasn't getting enough at first but this is all about the process of tweaking paleo to match your body and your needs. Lookup what people recommend and try to match that at first. If your not eating enough it can affect your energy. From what I understand as long as your eating the RIGHT foods, you don't need to calorie count or anything. Just eat good and until your full and everything else usually works out.

Starches? It was recommended to me when I had the same question that I should try adding a bit of starch. Maybe try a sweet potato with a meal and see how you feel.

Wish I could help more but these are just a few things i tried messing with. Its all about sticking with it and finding what works for you. I like to think of Paleo as a template for healthy eating where I can change things to match my body's needs. Good Luck!

The first time I got sick after changing to paleo confused me because I had no obvious symptoms, but I felt like what you describe.
I slept for a day. Didn't eat at all that day, and didn't eat much the next but felt well enough to go to work. By the third day I had an appetite back. I believe, if I had been on the SAD and following the standard advice about eating while sick, whatever it was could have easily lasted two weeks. Nassim Taleb has written somewhere about how it makes no sense to make sick people eat, and I think a few of the advocates of intermittent fasting have mentioned it as well. Processing food takes up resources better spent on the immune system at that particular time. Barring some sort of emergency, the body should regain an appetite

I throw this out as a possible answer because, well, it is possible that your symptoms have nothing to do with diet. You may even want to see a doctor, though I try to avoid them as much as possible myself.