First Off I've Had Anal Cancer In October 2013 .I've

First off I've had anal cancer in October 2013 .I've had all treatments and am cancer free now..I wear a colostomy bag but in my last surgery my doctor had to take out my small intestins because of all the radiation damage ..I've been getting really weak in my muscles in my legs the doc said my magnesium was less than half if normal..I'm taken mag.500 2x a day but I go so much during the day that my body isn't holding it in.what do you suggest I do?? Ask for magnesium patch or what I really don't know?? Please help