I have been getting sharp pain under my left breast (ribcage) for months now. Sometimes hurts when taking deep breaths. Heart related?

Unlikely. These kind of sensations are not rare. Very frequent actually. That said, you do need to get into the habit of doing self-breast exams. Same time every month, either in shower or lying down 2-4 days after your period ends. "lawn mower" technique advised. W/respect to cardiac worries, need an in-person eval to listen to heart sounds, get chest XR & 12-lead ECG, which may prompt echo & telemetry.