Why do I hurt????? 😞

So I will be 36 weeks pregnant very soon. I am 4 foot and 9 inches and probably about 125 pounds in total and recently I have been experiencing excruciating pain on my back my stomach and just my entire abdomen area anytime I’m briefly on my back. So for an example, I could be laying on my left side and then switching over to my right side and temporally having to be on my back for maybe five seconds and it hurts so bad every time. My baby girl is probably about 6 pounds now. Could that TERRIBLE pain be just because I’m caring a big baby and I’m a small person? Something more serious? It hurts so bad And it’s something that just started up this week. But it’s to the point where I almost can’t breathe. I struggled to breathe all of the time and people say it’s because she’s on my diaphragm, but nothing helps with it. I could be elevated, I could be laying on my side, I could be changing positions, but no matter what I struggle to breathe horribly, along with the terrible pain. I’ve never felt pain like that before pregnant wise and before pregnant.