Fertile already?

Hey ladies this might be a stupid question but I'm curious. This is the first time we are using opks at home. We've had a dr helping us in the past but want to try again on our own this time. My period ended Sunday(it was 4 days early & short), I had spotting yesterday and I took a opk this morning(drs order) and it's got two lines that I can clearly see. Is it possible to be fertile right after your period ending? I always thought it took a few days