As a boss, it actually makes me feel uncomfortable when an employee tries to tell me what's wrong with them; sometimes the more they talk the less genuine the story sounds. I prefer they just notify me by text if they are taking a sick day. We are all entitled to 7 per year and that's that.
Coincidentally, I have IBS myself and awkward as it was,

when the condition first flared 5 years ago, I did discuss it with my boss. It was necessary to do so because it was initially so severe it required accommodations to my job to relieve me of having to do site visits or take meetings away from the office. Over the years I've developed some coping strategies that work well for my situation and it very rarely interferes with work anymore. I do however take a sick day a couple of times a year if my acid reflux has caused a completely sleepless night.