Funny thing about that is I too have a very unusual birthmark in the center of my chest that I have been told my paternal Grandmother also had in the exact same spot. I often wear a large jade piece on a chain to cover it as it looks like a hole. As a teenager I told some rude boys who were teasing me about it that I had been shot. I said it as a joke (ok sarcasm) & they believed me so I waited over 2 years until just before we graduated before I told them it wasn't true. They stopped teasing me after I said it was a gunshot, and their friends laughed when they learned they had been fooled by a girl for 2+ years.
I hope its ok to share another weird story about my birth mark that happened later in my early twenties. Went to a Psychic Fair in Solana Beach, CA with a friend and had a Psychic I had never seen before approach me on her break to ask about my BM. After I told her my grandmother had the same exact birth mark she shared that as I walked by her booth earlier she had envisioned me with another very young girl who was close to my age at that time. She shared it was probably a distant past life we shared that she described as like an early Native American tribal community where we were tried and condemned to die by wooden stakes through our hearts after we were found mixing herbs, and plants we had been gathering to use for medicinal purposes. My Grandmother died when I was 6 but up until then she cared for me daily while my parents worked, I also share her name. Wasn't so sure I believed in past lives but it did give me chills. Then approximately 10-12 years later another Psychic shared the exact same vision she had about my birth mark. I did not know her either, nor had I mentioned anything about the earlier reading to her. Every hair on my body stood up on end & I had large goose bumps everywhere. Made me think these things could possibly have some truth behind them? Later trained with Elisabeth Kubler Ross who taught me past lives are very real and more common then most would believe.
As I read your post it made me wonder if a medium, or sensitive might be able to connect your birth mark with your relative's; or perhaps give you some other interesting information about it? Most scars, marks, or unusual anomalies are known to be carried from another past life traumatic event. That is from one of Dr. Weiss' excellent books on regression. http://www.healyourlife.com/authors/brian-l-weiss-md
Thanks for sharing such a unique topic here! :)