What Causes Irregular Periods With Little Bleeding?

Hi, i am a 26 years female. i am having issue of irregular periods. my period date is 21st but in September, i got period but only fir 1 day,it started normally but stopped after 1 day . i waited and then went to doctor for pregnancy test too but it was nill then in October,i got period on 14th that was normal. and now in November, its 21st and i dont have periods but from 11-nov,i am having little bleeding, as on 11th-i thought my period started but it was just a clot of blood and then from that day,little bleeding,very little or just a small clot of blood. i am very worried,what to do