Baby not very activeon Ultrasound and measuring small

I have had 3 M/C so I'm super nervous. I have had brown discharge for two weeks now my doctor cannot figure out why. My HCG is doubling but at my last Ultrasound I guess the baby measured smaller they said instread of 8 weeks I'm 6 weeks. I thought no big deal until my doctor called. He said the baby wasn't moving much, they saw the fetal pole and yolk sac. I don't know what to think, I go in for another Ultrasound in 2 Weeks but I'm going crazy... Anyone been through this?