How serious is having left kidney pains on normal days, and while on period, and after drinking water? and my right one also hurts but not as much.

See remark: This is unusual unless you have partially obstructed LT. kidney,specially when you drink water which increases the load on the kidney. Given your young age,I recommend you see a urologist in your country to evaluate and manage the problem. As a urologist ,this is what I'll do in this country. I'll be interested to know the outcome . Best in Health and ma'a salama ! JBR