Mass Behind Placenta

Jettscy

I went in for a ultrasound today to check my fluid (it was borderline low) and the tech saw a mass behind my placenta.  The Dr. said she couldn't tell if is a blood vessel or bleeding.  I'm now in L&D hooked up to a NST overnight.  My Dr. came in and said since I'm only 37 weeks and 4 days so they are going to send me home since the baby doesn't seem to be in distress.  I'm now worried that it is bleeding.  I would rather get the baby out now then to have something wrong with my placenta.  I was wondering if I should tell her when I see her tomorrow that I don't feel comfortable going home if there could be a risk that something is wrong.  Am I panicking for no reason?  What would you do in this situation?  Any help would be greatly appreciated.