could be a lack of minerals. i would add a good mineral source to your fast like black strap molasses or sea vegetable broth or 100% pure coral calcium. this is my favorite mineral supplement, its fantastic.

http://www.active-liquid-minerals.com/contents.htm

minerals should be taken with a natural acid like lemon juice or apple cide vinegar to dramatically increase bioavailability.

Hi, Carrotgirl, do not worry too much. I experienced a lot of pain on legs and joints during the juice fasting. After the fast,I feel so much flexible on my joints. I can stand up for a very long without feeling any pain.
You must be sure to drink enough liquid (juice+pure water) and have BM every day.

Now it is end of my day 4, I have headache all day long and start feeling pain here and there. It is a learning journey and you will become more confident and know more about amazing healing power of your own body.

wish you go well,
peace and blessings

I read in my juice fasting book that muscle pain is normal during the initial days of your cleanse because a lot of toxins are being released into your muscles. I experienced this in the muscles in my back during my first week and then they went away.