What Causes A Delayed Period With Negative Pregnancy Test?

hi i am 20 years old girl. i am stying with my boy friend since last year. its already few days i dint have my period. i have used pregnancy test kit and got negative results. last month i had period in 9 of august but today its 15 sep its not there yet. what should i do. where can i go and consult abt it. i am living in pjs area. actually i havent been sleeping and eating well these few days. is it because of stress that my period has stopped. Incase i am pregnant can i use medecine. plzz help me.