Aw first time mom, that sharp pain under your breast especially when sitting is actually pretty common pero hindi dapat ignored, ha, a few of us had that too and it turned out to be different things para sa iba (muscle strain, trapped nerve, sometimes even how we're positioning ourselves habang nag-aalaga). Better have it checked with your OB or regular doctor kasi they can properly assess what's causing it, especially since it's really painful for you na, better safe than sorry, lalo na first time mom ka pa lang at stressed na enough! Wishing you feel better soon, mommy.