Breathless

Whenever I lay on my back I feel like I can hardly breathe.. its so uncomfortable...

So whenever I lay down on my back (whether it be in bed, on the couch, or at the OB office for measuring) I struggle to breathe and my chest feels "heavy".. does this happen to anyone else? I would like to relax and elevate my feet to reduce some of this swelling, but I can't do it laying on my back (any suggestions would be great ladies, thanks!) *Currently 35 weeks & 4 days and this has been happening for several months*