Late Rabbit pregnancy?

One of my rabbits first had a litter of 8. Two were stillborn. The six that survived are exactly five weeks today. However, a friend of mine put one of the male bunnies back in her cage..her last 3 days of pregnancy, thinking the buck was "lonely". It became pretty obvious that she was pregnant again, two weeks later, when her belly was huge and her diet hadn't changed. Today is the 37th day of pregnancy. I didn't get the dates wrong. I wrote down the date her first litter was born, and obviously since she got pregnant again right after giving birth..This is the 37th day. I can still see babies making wave movements around her stomach..but she hasn't tried to make a next. Typically she doesn't make a nest until after the babies are born, but why has she not given birth yet? Are the babies going to be okay because of it? Is she holding off giving birth because she has another litter? Is there anything I can do to coax her into it?