I Have Had Diarreah Since I Was In Costa Rica

I have had diarreah since I was in Costa Rica January 2015. Can't seem to get it to stop for more than a day or two where I have a fairly normal bowel movement. My MD ran tests and said I had nothing, but, when he put me on an antibiotic, I got better for a few days. I also get cramping and gas on occasion. Recently I am getting lower back aches and some nausea. I have noticed that milk makes my stomache uncomfortable.
I take psycillum husks in capsule form.