Swollen feet vs. can't lie on back?

So I'm 24+4 and I've noticed my feet swell up when I'm doing just a fair amount of standing and walking throughout the day. My mom says this happened when she was pregnant with me, and she was told by her doctor in the 80s to lie with her feet elevated above her head daily for at least three hours. When I tried it last night, all I kept thinking was how I've read repeatedly how I should not be lying on my back at all at this stage in pregnancy since the weight of the uterus slows oxygen to my baby boy. My next appointment is tomorrow so I'll ask my doctor, but does anyone know which too here is correct and which is harmful? I'd love to rest on my back for hours at a time but I thought I wasn't allowed. Thanks!