What Causes Stomach Pain After Taking Antibiotics For Sinus Infection?

I had a sinus infection and started taking an antibiotic. During this time I started having terrible stomach pains. i am done with the antibiotic but I still have pain around my belly button. Three week ago I also had an IV injection of RECLAST. I started taking Advil because of the side effect. My stomach is a mess. I have terrible stomach pains before I go the bathroom. The stool is pale yellow and green at times. Sometimes it is light brown. I am 60 years old and I am worried. Also I have nausea when I need to go to the bathroom. Thank you.