How to lay while sleeping

T
So I'm in my third trimester. I'm 30 weeks 2 days. I can only lay on my back when relaxing or when sleeping, but my friend told me it cause me and my baby to loose oxygen. Laying on my back is the only way I'm comfortable. I was told by my friend that I should be sleeping on my side but it hurt like some one is pinching me when I try to sleep that way. Someone please tell me what should I? Aldo's sleeping on my side hurts my back and side more like I'm getting pinched but when I'm sleeping on my back with my body pillow I'm very comfortable