Hello , one week before my period, i notice a little red pimple next to my left nipple . sometimes it become flat . what should i do . thank you ?

May be normal: If this is just a small pimple next to your nipple, it should resolve on its own over time. If it doesn't, an exam is in order so that a good diagnosis can be made and a decision as to whether or not anything needs to be done and, if so, what. If it starts growing or becomes painful it should be seen by a doctor sooner. Hope this helps.