Shortness of Breath

Hello, mommas! I am currently 23+3 with my baby girl and I have been experiencing Shortness of Breath lately. I have asthma and I am an ex smoker. I’ve been told that it is normal and usually starts in the second trimester. However, I just want to reach out and see if anybody else is having this issue? I own a pulse oximeter and have noticed my oxygen level tends to drop sometimes when I am laying down. I have to prop myself up to keep them at safe levels for my daughter. Anybody else have any similar experiences?