It only smell when i pee when my monthly is on?

As long as there: is no pain, increased frequency or fever it is not worrisome. It is likely due to hormonal changes and blood.

Blood: You are likely smelling a slight discharge of blood when you are urinating from your vagina during your period. Nothing to be concerned with unless you are experiencing burning with urination and thus then it may be a smell from your urine and possible urinary tract infection for which you should see your physician and get tested and treated. Best wishes to you.