What Causes Dark Brown Discharge In Pregnancy?

hello,im 24 years old i weigh 70kg and my height is 5 9 ,im 15 weeks and 5 days pregnant.when i was 10 weeks pregnant i had some bleeding i went to my doctor and she asked me to take bed rest for about 2 weeks i was all better ,then again 12 weeks i started bleeding went straight to the hospital and was admitted for low lying placenta and was told the bleeding was because of slight detachment.i stayed for 3 days in the hospital and was all fine for next 2 weeks and on complete bed rest few days back i started having dark brown discharge,it keeps coming on and off and is alot when i wake up in the morning.my question is what is the brown discharge ?and does the placenta attach to its place ?