My Doctor Want Me To Take Medroxyprogesterone 3 Weeks After

My doctor want me to take medroxyprogesterone 3 weeks after my period can you explain to me why and also l have A.U.B I was also put on birth control pills and have breakthrough bleeding for 3 months so she took me off and told me to take the medroxyprogesterone can you please help me I don't understand why she would give me medicine that would make my bleeding worse thanks.