brown discharge

I'm 30 weeks today with twins. I still have a marginal placenta previa. I started bleeding heavily and went into labor at 24 weeks so I've been on bedrest ever since. I've had brown discharge 3 days in a row now when I wiped. I went in earlier this week and they didn't seem too concerned. My cervix length was 2.6, but they wouldn't check to see if I was dilated as it could put me into labor. Has anyone had this in their 3rd trimester? could I be losing my music plug?