What makbike said, and try to consciously relax when you exhale. It's hard to explain but sort of like letting your belly fall. It's helped me before when I had side stitches. And then when they're gone, try to breathe regularly and not "straining", just controlled relaxed breathing.

AFAIK side stitches are a cramp in the diaphragm - maybe the meds are dehydrating you or something, try some water and magnesium?