Since it's your lunch break, is the problem that it will run a bit over into work time? If so, you might just say you have a standing appointment on Mondays (or whenever) during lunch and because of travel time it will edge a bit into work time, but (maybe) you'll make up for that time by coming it a bit early or staying a bit late. Hopefully that will be enough. If they press, just call it a doctor appointment for "monitoring" and that you'd rather keep the health reason private. They can't really press beyond that.