Decreased fetal movement and lower afi

Mariah
So over the last 2 days I have barely felt my karate kid move. She was so active throughout the pregnancy and just 3 days ago I felt her whole body shifting around. I started noticing a pattern with her since I'm now at 31 weeks. I've tried drinking something cold and sweet, laying on my left side, eating everything she would normally respond to, shaking her up, poking, rubbing, and barely I'll get a nudge here and there. So I called l&d and decided to go in. Immediately I heard her heart beat and was relieved. On average it was about 166 but today 145. They did an ultrasound and I saw her move her little fat feet but she's not active how she was before. The afi was 10 which I've researched is on the low side. They sent me home and said she's ok. Between the afi and her heart beat I'm still a bit concerned. The last measurement sonogram was around 24 wks and she weighed 3 wks ahead if she still on that track she's pretty big right now. Could it just be she's too fat to move so much now, like she ran out of room? Should I be concerned?