Possible issue

I'm 30 weeks pregnant & I've been having "breathing issues". I know shortness of breath is common, but, I would be sitting up and all of a sudden feel this pressure on my chest that causes me to struggle breathing. This also happens when I lay on my side. It happens everyday atleast twice a day & im worried. My doctor checked my heart & lungs today and said everything seemed fine. She did no further testing. What could be causing this besides the basic third trimester symptoms. Something seems off... Opinion? Please help.