How heavy is the flow (do you need a regular tampon or maxi pad to catch it or juet a pantiliner/lite tampon)? There are some normal things that can cause this (breakthrough bleeding on birth control
, missing birth control, ovulation spotting, being peri-menopausal or having just started having your period). There are also a lot of problems that can lead to this (endometriosis, hormone disorders, miscarriage, etc.). Would need more info on your problem to give a good answer.