What can I try to sort this out?

I have for some time been suffering with frequent Diarrhea.I take Lithium (priadel 1200mg at night) and have been tested recently for thyroid,potasium syrum level etc.I have had bouts of muscle weakness in my legs that are seriously hampering me in my sport of long distance running, I lose all stamina and cant perform or run very far.

I took creatine recently and the problem got alot worse so I stopped it.Running means a great deal to me and has helped me deal with my bipolar condition and quit smoking when I was 30 and give up alchohol completely.I have only been running for 4 years and last year I was 64th in Wales at Half Marathon but I have not been able to progress.

I dont beleive it is overtraining as I have cut my mileage right back but dont feel I can increase it untill I deal with this problem.I take a good multi vitamin and try to eat well and I have increased my fluids which has helped but the Diorrhea will not go away.I have an apointment next month on the 3rd to see if its coeliac or crones and am considering a full body mot test privately after that.I am male, 10 stone, 5 foot 9 and 34 years old.Could you give me your feeling on what this problem could be please.