I can't answer this for Omicron however for delta and original strain, it took 3 days on average to become contagious and 5 days on average to become symptomati

I can't answer this for Omicron however for delta and original strain, it took 3 days on average to become contagious and 5 days on average to become symptomatic/test positive with a RAT. (There are extreme examples of it being 14 days). It is more than reasonable that you were infected after the booster and before it was fully effective. The vaccine is reported to take 14 days to be considered to be in full effect.