31 weeks, light headed and can’t breath every time I bend down or lay down.

I get annoyed seeing all these women on social media working out or doing crazy mom things, and I can’t even lay down or bend over without an intense feeling of dizziness and like my lungs are being squished. My OB states all of my blood work is normal, and I’m not anemic “enough” to need iron supplements. I also drink a ton of water and snack throughout the day. Any idea why or is anyone else like this? I have a demanding job so I’m constantly walking and bending, but you’re not about to see me weight lifting.