What Causes Prolonged Bleeding During Periods?

I ve been on my period for about 3 weeks now... I usually don t have them after having my son 17 months ago but I started back around 2 months ago.. Well this time I ve been on for 3 weeks and now I m only bleeding at night and none at all during the day... I was also diagnosed with cervical cancer but they said it went away.. Any tips or advice would help because I m freaking out