So I've been doing paleo for about 3 weeks and I love it-I have much more energy and I'm generally in a better mood....the only drawback is that I've noticed my muscles are much weaker. I can't do what I used to do at the gym and there is a big flight of stairs that I go up almost every day that used to be tough but I'd only be out of breath-now my muscles are on fire and so tired when I climb them. This has only started happening while I've been on the paleo diet...anybody know what it could be?

Quite possibly you're still adapting to ketosis.
Try having some fruit before the gym - I find a kiwi fruit and a banana provide more than enough glucose to get through a weights session.

Options:

1) You are not eating enough. That has been the case with people who switch to paleo, remove all the non-paleo carbs they had been eating but neglect to replace them with enough fat. Run some numbers on what you are eating for a few days. Try to eat more and compare how you feel.

2) You are eating enough but still adapting to ketosis. This should sort itself out some time within the first 1 - 4 weeks.

3) You need to fill your muscle glycogen more actively. Eat some fruit after physical activity.