I was on Cellcept for quite a while (it turned out to be a miracle drug for me, my Lupus has been 'quiet' ever since). I had some minor side effects - hair thinning, sometimes, like Melissa some explosive diarrhea, but nothing like you describe.
I would definitely go straight back to the Dr ASAP - what you're experiencing is well beyond any side effects of Cellcept I've ever heard of.
Lynnwood, Dx: 2000, Found HW 2005, HW Lupus/Sjogren's Moderator May 2006-May 2018
DIAGNOSING LUPUS & HW's LUPUS 101
"Life is far too important to be taken seriously." - Oscar Wilde