Anytime you have pain while taking Lithium that comes out of no where, you should seek medical attention as soon as possible. I am on lithium and at one time was taking 300mg (was too much for me personally) and suffered lithium toxicity. My muscles were cramped up and I couldn't move very easily, I felt like I had the flu, was running a high fever and couldn't hold food or fluids down very well. Went to the doctor and they ran a test and found out it was the Lithium. I was then decreased to 100mg and it has since worked great with me. I had the diarrhea problem for the first month and then was told to take over the counter pills that pretty much stop the diarrhea and give you regular bawl movements. I can't recall the name of the medication, but it's in the same area in stores that laxatives are, I believe. I took those for about a month and after that, I got off of them and since have had regular bawl movements on my own. So maybe that's something to think about as well.

Whatever the results may be, I wish you the best and hope you can find something that works for you.