Some times I get a sharp pain underneath my right breast. Specially when I take a deep breath.  unbearable sometimes and I try to breath slowly.?

"Stitch" If the pain occurs only when breathing in, or is much worse when breathing in, it is almost always due to a fold in the pleura (knife-like pain) and is a non-problem. A very deep inhalation will generally smooth out the fold (though it hurts).
? Pleurisy. This can be pleurisy and warrants further check by your doctor.