Is that gas or what?

I am past 20 weeks pregnant and I still really don't feel my baby move! What the heck? I felt my other ones move at around 18 weeks. My placenta is anterior so that may be part of the reason....or that I have a ton of fluff ........I don't know! But, I am seriously annoyed! Once in a while I will feel a little flutter....and I think....." oh i think that was the baby....and then a few seconds later I can actually hear my belly rumble....." oh....Gas again...: ( " I feel like I really should feel my baby move by now......I should be able to tell if that flutter is my baby or if it is really truly gas. This is driving me mad! I mean....this is my 4th baby. I should be a pro at knowing what is what! I am hoping that in the next couple of weeks I will def feel some baby movements. Come on baby! Kick me hard!!!! Please!!!!!