US & reason for spotting!!

Garratt
So I'm 6w3d and for the past week or so I have been spotting. I have been worried sick. I have a doctors appointment tonight but I knew they would make me wait at least another couple of weeks for a scan so today I went for a private scan.
First of all it looked as though the sack was empty, I almost cried but then hiding in the (bottom left) corner we saw a little flickering heartbeat and there was my beautiful baby!! I'm so happy.
They also found a 7mm blood sac under the gestational sac (you can't see it on the pic) this has been causing the spotting. They said it shouldn't be a problem unless it suddenly gushes out and flushes the baby out with it. This is obviously very scary. Has anyone had any experience with blood sacs ??