Aww mama, pain sa lower breast area can be from so many things, muscle strain, bra fit issues, or sometimes hormonal changes, lalo na if FTM ka pa, pero dahil grabe talaga ang sakit mo, better check with your doctor para sure lang, kasi gusto natin na ma-rule out anything serious. One of my friends had similar pain and it turned out to be a rib issue na na-aggravate lang ng wrong bra size, pero ayun, kailangan talaga ng proper check-up para malaman mo exactly what's going on. Sana mabilis ma-resolve ito para sa'yo!