Why does it hurt to have sex with my boyfriend when im on my period but only if he hits a certain spot

During menstruation, your cervix may become engorged. Your cervix is the passage between your vagina and your womb, and it may become more sensitive due to hormonal changes. The cervix is found near the end of the vagina but sits almost at right angles to the direction of the vagina.
The advice here is that if you feel any discomfort during sex, you should stop and wait a day or two. If the pain is still there, you should see a doctor.