Can Nuvaring Be Started With Missed Period And Negative Pregnancy Test?

i have not had sex for 6 weeks. I have not gotten a period since my last time of having sex which was unprotected and I took Plan B after. I bled after that time. I had an ultrasound and the doctor said I had fluid trapped at the top of my uterus and that my lining of my uterus looked thick like my period was coming...has been almost 2 weeks. I am concerned about when to get my period. I am also waiting to start my Nuvaring until my period starts but would really like to start it now. Can I start it now if my pregnancy test has shown negative....?