how do I tell work...

So I've been lucky because my last fet was scheduled on a day that I had off from work and I luckily had the next day off, but it didn't end in a pregnancy, so I start my second fet cycle tonight, I go in for blood work on the 28th, so if everything looks good my fet will be October 3rd, the nurse at my clinic said there's no way to know for sure if that will be the exact date until the results on the 28th... how do they expect me the give my job a 3-4 days notice that I will need a few days off! With all the stress this tops it off! What do I tell my boss, she already gave me a hard time when I told her about my egg retrieval back in June! I'm so worried! I don't know what to do! Anyone have any advice...