Hi,
     I understand your concern.
   *  - Fetal movements get reduced during 3rd tri master of pregnancy( 7th -9th month) as baby tries to move down & enters pelvis. This makes restricted place for fetal movements.
     - with advance of pregnancy, daily routine of baby starts in 
uterus. May be when you are free to note baby movements- it's babies sleeping time.
     - Maternal 
stress with illness/weakness may affect baby movements. 
  * low lying placenta/ macdonald's stitch do not affect fetal movements.
  * You sat that USG is normal- that means babies 
heart sounds are normal. You need not worry in that case.If you are reffering to an old 
sonography,please visit your doctor soon & get the matter evaluated.
  * get babies heart sounds counted every 3 days for 15 days & gain confidence for fetal well being.
   - Lie down in left lateral position & try to feel baby kicks.
   - taking 
cold drink/juice with some sugar this may stimulate your sleeping baby. 
   Thanks