I'd wait a day or 2 after taking a shake to do a liver cleanse...but, frankly, I'm not sure how it will go. I've never done a liver cleanse in the middle of a Colon Cleanse so I'm just not sure. I can't think of any reason there would be a problem provided your system is pretty clear of solids so that the stones, etc. can pass right through. The Epsom Salt flush would certainly take care of that.
BTW, your pain could also be radiating from your colon...although the Liver Flush would probably be a good idea, too. I've gotten rid of aches and pains in all sorts of places by doing a Colon Cleanse or giving myself a colonic. There are some theories that there are reflex points in the colon that correspond with all different areas of the body...much like reflexology of the feet or acupuncture points in the ears. It's all connected...