I've been having a dull pain in my left breast in a localized area under my nipple it happens at all time of the month, not just my period what is it?

Needs to be checked: It is very difficult to say what is causing your constant pain in left breast without examination . Since it is there all the time and not related to your period and you are only 22 years old. It may be due to an inflammation, or due to a cyst etc if it is only localised in the breast . I suggest you get it checked by your Family Doctor who will examine your breast&may need ultrasound imaging