freaky fetal movement

I'm 36w5d and baby is always moving. She's starting to slow down but she's dropping and I'm sure she just has no room. So through this pregnancy I've figured out what almost all the different movements are- her readjusting, her hiccupping, her drinking amniotic fluid, etc. But there's one thing that I can't figure out. It's like a spasm or shaking like the baby is having a seizure in there. It only lasts for a second but it's pretty violent and it's starting to freak me out. It's been happening randomly for the past couple months, maybe longer. Is this normal or happening to anyone else? Anyone know what's going on in there?