Back pain while pregnant so bad I fall down

I’m 29 weeks pregnant and I have horrible pain in the lower left side of my back.

I know back pain is 100% is normal, but sometimes it’s so bad that my left side will literally give out on me and I’ve actually fallen down. If I even move the wrong way it feels like a knife is going right into my back.

Has anyone else had it that bad? Did it turn out to be anything? My doctor doesn’t seem too concerned but I just feel like it’s worse than it should be.