Irregular Periods With Reduced Bleeding. Taking Advil Or Tylenol. Not Pregnant. What Are The Other Reasons?

Hi Doctor, I am female 31 years old, married. No kids yet. I always have regular periods i.e exactly after 28 days. May be once a year it just comes early after 15 days or so. This time I got after 24 days(even last month). Bleeding is normal like usual. Started on 10th and finished on 14th. usually on the next day i have very little bleeding and it ends. Always. But this time, even after 5 days I see minor bleeding/spotting. Mainly Red. During the day, 3 times I see this when I go to bathroom. rest of the times its fine. I am keeping the pad on. I am having period from age 12. But this never ever happened. I took Advil or Tylenol during 1 and 2 nd day of period. please help me know what happened. I didn t have intercourse during or after period. pregnancy is not possible. I have never taken birth control pills. So what else it could be?