I was sexually active before my period and i took the contraceptive pills, my period came a week after but it was so light and on the fourth day i had?

Your hormones: are likely in flux for whatever reason, and you would need to be patient to see if things would settle down. Menstrual fluctuations are affected by many different factors, including nutrition, psychological state of mind, stress, infections/ illnesses, etc. If things continue to be irregular or unpredictable in the next couple of months, get yourself to a GYN or family Doc.

Morning after pill?: When you say you took contraceptive pills after having sex, do you mean in the dosage for emergency contraception? Or were you on daily contraceptive pills when you had sex? Also, I'm not sure what happened on the 4th day, but I suggest taking a pregnancy test, and repeating it a few days later. And then get on a reliable contraceptive, either daily pills/patch, IUD, or an implantable.