My girlfriend has been bleeding a couple days after her period for the past couple months but her period is normal why could this be happening?

Is it normal? No.: Bleeding in between periods "inter-menstrual bleeding" is not usually normal and can have different causes. Does the bleeding occur after sex? Is sex painful? Are there other symptoms eg. discharge? The best course of action is to see a doctor for cervical smear, vaginal swabs, examination and further assessment to ensure that there are no abnormalities causing the bleeding.

Intermenstrual bleed: Is generally not considered normal and needs careful assessment for underlying cause (pregnancy related condition, infection, cervical problem particularly if history of abnormal smears, thyroid disorder among others). Please see a doctor licensed in your area for further evaluation as you may need tests run and then they'll be able to advise on treatment/management options.

Residual blood: There is probably residual menstrual blood in the vagina that has been stored after the uterine bleeding has stopped. Once that occurs, the residual blood "dribbles out" over the final two days.