What Causes Pain During Urination While On Antibiotics?

I am on a flu and other antibiotic and I am having sever cramping and it hurts with cramps to urinate. I have not have a bowel movement yet but I am scare that will hurt like crazy. I did take Imodium when I have runny stool as my doctor suggested but that was on Friday. What can I take to help? I did have some cranberry juice this morning because I hear it helps and I will have yogurt this evening, is there anything else I can take?