Hi! i'm experiencing severe pains on my left shoulder and back before & after my period. could it be due to my endometriosis?

Yes: Though refractory pain in the shoulder would be a little high for endometriosis, one cannot rule it out. I would recommend a follow up appointment with your obgyn to discuss treatment. Typical first line treatment are non-steroidal anti-inflammatory medications. See a physician if you haven't already, though, to discuss these severe pains.