I had one case of diarrhea on the first day of my combination birth control inactive/placebo week. does the diarrhea affect my protection?

Unlikely: The pill contents are absorbed from the upper intestine soon after ingestion. Diarrhea is a failure of your large intestine to extract water from the fecal debris. Unless you were running to the BR every half hour & after any oral intake it is unlikely it would effect your pill performance. Normal transit time-mouth to end is 3-4 d/normal 1-2 d simple diarrhea.