I second what Velouria said. I would tell them something very vague, such as "I was having some medical treatment." If they press, "I'd rather not discuss it. Thanks so much for asking." If they press, just keep repeating the same answer. I've used that in other situations. Works like a charm.

At my work, people sometimes go on leave for medical procedures or treatments, like cancer or a hysterectomy. They don't discuss it with anyone. None of anyone's business. Same for you.

Hope it goes OK.