Baby movement.

dancekatidance

So tomorrow I will be 21 weeks, and the past few weeks I have started to feel movement as most people do, however I noticed that It's a lot more difficult to feel her when I am in an upright position. Is there a reason she only likes to moves when I'm laying flat? I know that they say when you are up and moving around the baby is usually sleeping, but I'm talking about when I'm just sitting upright in bed or on the couch.