What Causes Irregular Bleeding During Period?

hi um well my period was a week late and then i got my period it was 3 little spots then it stop and started back up 3 days later. it first would stop and go thur out the day still very light not enough for a pad but liner, this lasted for 2 days there is no pain no nothing the is very usual because i usually feel alot of pain and not in the case. then after the 2 days it seem that it still light and medium . its a dark color and sometime a bright red. am 23 do you know what it could be i took a pregnancy test and negative a little worried