有可能是着床出血，但是5天就着床好像太快了

Normall it takes 8-10 days for the fertilized egg to implant in the uterus. But it's not impossible for some people to take only 5 days. You might want to take a pregnancy test when your period is due this month. We can have midcyle bleeding/spotting for a lot of reasons. Don't worry too much.