No, you're not the only ones: periods CAN affect your digestive system. During your cycle, your body increases its production of compounds called prostaglandins and decreases the output of progesterone.
Since progesterone is needed for pregnancy, levels of the hormone increase when your period begins. When your body realizes you are not pregnant, levels drop back down, while the prostaglandins increase. This causes contractions and thus, some stomach issues, or its scientific name "period poop." (source: @bustle)
via @comingsoonny