What Causes Painful Periods?

Hello, I am a 17 year old girl. I had sex for the first and only time last September but am definitely not pregnant as i have taken pregnancy tests, had my period, and got a sonogram to check for ovary cysts. But my periods have tended to be very painful so my doctor put me on a low dose birth control. Up to a month or so before starting the birth control a white milk like substance has been coming from my nipples. Could this be lactation? and could it be due to the fact im living in a house with two babies i help raise one of which is a newborn?