4 days very light red spotting when wipe, didn't miss only bc pills took plan b since 1 st mth on bc, pg test neg, normal for period to be this light?

Irregular periods are one of the most common problems Obgyns treat.: If this is one time problem it usually resolves on its own. If this is a recurring problem you should be evaluated. Infections &  hormonal changes can cause spotting. If there is a concern for pregnancy then do  a home pregnancy test .  there are many options available to help regulate the menstrual cycle. Schedule a visit.