7weeks ultrasound they found bleeding

I went for my ultrasound Monday and the baby and heartbeat looks great but outside the baby sac (right next to it) they found a little bit of bleeding and put me on bed rest I'm just wondering what this could be they didn't tell me and when I asked what gonna happen if it doesn't go away the nurse told me lets just pray it does so now I'm upset and worried any ideas what this can be??