Hi codomo,
If you can trust your boss to keep it confidential, it is easier to be direct rather than every time have to think of different reasons and excuses.
However, if you really want it to be private, just say it's a minor op. If she keep asking and probing, keep saying it a personal issue and you rather not reveal. She should be able to respect that. Another option for getting her off your back, without lying, say "it is a gynae-related issue, the details are quite technical". If next round, hopefully no need, you can say that you need a follow-up proceedure. And usually the spacing may not be immediately one month later.
IVF protocol varies depending on clinic, dr and individual.
I can share with you my protocol, known as the long protocol. After AF start, visit dr & scan, do nothing for 21 days (some dr will give birth control pills). On 21st day, start to inject lucrin (suppress your hormones). Then, AF should come again (timing varies between ppl). Visit dr for scan again and then when the time is right, inject follicle-stimulating hormones (eg. puregon/ gonal-f, etc). There will be scan to see if the eggs are mature (like your IUI). When the size is large enough, they will inject HCG and 36 hours later, perform egg retrieval. Then collect sperm, fertilise the eggs. Some will choose to implant 2-day, 3-day embryo, or blastocyst at 5 days.