I have found a small hard ball in my right breast, it moves around easily. My period is due. The lump is well defined and has smooth edges. I'm 24.

See your doctor. Unfortunately even in the most experienced hands, feeling a lump really tells us nothing. Best to see your doctor and discuss if any imaging recommended. Your family history re breast cancer important as are other risk factors. Your age argues against significant concern, but I have also diagnosed breast cancer in women in their 20's (rare but does happen). Please be examined/talk with your doc.
Breast "mice" Likely a benign lump, monitor for change in size, See https://en.wikipedia.org/wiki/Fibroadenoma.