Please go see your doctor or head to A&E now to get this checked out. At 38 weeks, any noticeable change in baby's movements is something that really shouldn't wait, even if baby is big. Better to be safe and get assessed properly so you know everything is okay.
My baby movement also got slightly less, used to make me so anxious. Try to drink really cold water, have something sweet. As long as you feel something small. It’s all good