weird discharge! help!?

I'm 15 years old, and I just found really odd vaginal discharge. it was very white and elasticy almost, it would stretch a little, but not much and it didn't break apart easily. i had sex three days ago and he came inside me on accident, but he got me plan b and a pregnancy test. the test was negative (we were worried about the weeks before and we wanted to know if plan b would be worth it). im getting put on birth control soon, so i can get it checked out in about a week. my period is 4 days late now but i am really stressed from finals at school. does anyone know what this is?? :(