It cud b one of several things:
a) r u on the pill? Sore breasts is a common symptom,
b) cud u be pregnant? This again is a common symptom,
c) this is a typical sign that ur period is due soon due to the change in ur hormone levels,
d) do u have any sort of lump or swelling? If u do,get it checked out straight away by ur doctor,
e) r u wearing properly fitted underwear-this cud have something 2 do with it,im not sure.
If u r worried & u cant put the pain down 2 any of the above,maybe u shud visit ur doctor.
Liz